ZergSmasher has pretty much hit the nail on the head (my own fault for being so slow at typing) - much of the advice here is the same as above.
That first guy sounds like someone to avoid playing with, though I had similar things happen when I started playing
It is indeed a legal army, and quite often the Dark Vengeance set is used as a base for a Dark Angels army, so the following advice is quite generic and well-worn. Apologies, it's also quite rambly!
Did you end up with the rulebook from the dark vengeance set as well? Or did you split the chaos side with someone? Anyway, if you did get it, or are borrowing your friend's, on page 122 is a picture of the simplest and most well-known way of building an army - the
Combined Arms Detachment. There are way
way more ways of constructing armies, but we'll stick with that one for the moment.
You've a number of options with the tactical squad - you can keep them as is, at 10-man, allowing them to split up into two units just before the game using the Combat Squads rule, or just sticking as a single unit.
If sticking as a single unit, the Plasma Cannon will slow the rest of the unit down - it can't be fired on the move, and the bolters / plasmaguns want to be within 12" to do the most damage.
If splitting them up, leave the plasma cannon with four bolter-wielding buddies in your deployment zone (preferably standing on an objective), and send the other half forward.
Either way, the best way to make the best of a tactical squad is a transport - a Rhino, Razorback or Drop Pod. This allows them to move into optimum range and/or capture objectives quicker. Each of the three has advantages / disadvantages. The Veteran Sergeant upgrade is almost the same as an entire marine, but recently I've been managing to roll alot of nines for leadership tests and regretting not taking the upgrade... I prefer to equip my sergeants with a combi-weapon, but we'll stick with the pistol for now (it's pricier, but gives a bonus attack in melee).
The Scout Squad is fine, though they could use the Camo Cloaks upgrade (this improves their save when in cover).
With the bikes, I'd suggest different special weapons - a
RW biker with a plasmagun costs about the same as a Black Knight, who does the job a whole lot better. If you're able to get the spare part to swap, replace the plasma gun with either Meltaguns (to kill tanks) or Gravguns (to kill monsters or enemy terminators).
The heavy weapon in the terminator squad is an Assault Cannon rather than an Autocannon (more shots, less range) - you might find it difficult for them to earn their points back if your opponent has too much AP2 weaponry (as it will ignore their armour save). They also have the option of teleporting into battle (the Deep Strike rule is included with all Terminator armour, see page 154 of the Dark Angels book for more info), but this will mean they won't turn up until turn 2 at the earliest - in this size game that would leave you without a sizeable chunk of your army. Best to start them on the table in my opinion.
A Psyker (like your Librarian) rolls at the start of the game to determine what magical powers they know - there are 7 tables available to you (more if you get the Angels of Death supplement cards), and each has 7 powers (numbered 1 to 6, and one called the "Primaris" power). A Psyker rolls once per mastery level, and can choose a different table for each roll (i.e. a mastery 2 psyker could have a power from Telekenisis and a power from Pyromancy), he can swap a power for the Primaris after seeing the roll. If you roll all your powers from the same table, then you get the Primaris free anyway (because of the Psychic Focus rule).
Each turn you roll a dice and add your mastery levels to determine how much "charge" you have. You must spend charge to cast powers, so increasing the Librarian from a mastery 1 to a 2 not only gets you another power (meaning more options), but also increases the amount of energy you have available to use.
I'd also suggest a Conversion Field for him - Librarians are the only one of the space marine hero-archetypes (Master, Chaplain, Librarian) that doesn't come with an Invulnerable save basic (which is a save that can be used when your armour and/or cover save are being ignored), but Dark Angels have the advantage of being able to buy a decent
inv save for him.
Both the Master and the Librarian are Independent Characters, which is a special rule allowing them to join other units (so that they don't get horribly picked on by the enemy the first opportunity they get), and it's worth thinking about where they're going when choosing wargear and planning psychic tables to roll on. The Master model you have has a combi-plasma (it's slung on a strap, partly hidden by the cloak), which would mean he's best buddy-ing up with the Tactical squad (try keeping similar weapon profiles together, a unit can only target a single enemy unit per turn
normally*). Artificer Armour increases his save without the advantages / disadvantages of terminator armour (terminators cannot be transported by a Rhino, for example).
Are you able to borrow models to bring up your army? Or Proxy-ing (i.e. using one model but pretending it's another -
"This model is carrying a plasmagun, but I'd like to try out a gravgun on him if that's ok? I've paid the correct points for it.")?
*Deathwing have the Split Fire rule, allowing one of them to target a different target to the rest of the unit*
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So, if you've made it this far:
Company Master, Artificer Armour, Combi-Plasma, Relic Blade, Digital Weapons, Meltabombs (goes with Tacticals).
Librarian, Mastery 2, Conversion Field, Digital Weapons, Meltabombs (goes with Terminators).
Tactical squad (10), Veteran Sergeant (Plasma Pistol & Meltabombs), Plasmagun, Plasma Cannon (combat-squadded). Rhino transport.
Scout Squad (5), Veteran Sergeant, 4x Sniper Rifle, Missile Launcher, Camo Cloaks.
Ravenwing Bike Squad (3), Veteran Sergeant (Meltabombs),
special weapon or two.
Deathwing Terminators (5), Assault Cannon, 1x Chainfist.
This totals out at 934-939 points, and includes a fair few upgrades that could be dropped in a leaner list (digi weapons, vet sarges). It also needs a Rhino to transport half the tactical squad - If you go out and buy one new, get a Razorback instead, it's only slightly more expensive and if you don't glue down the main hatch you can swap between the two very easily.
For bukling the rest of it up, if cost is your primary worry, search internet auction sites for another set of the three
RW bikers from the
DV set, some melta or grav guns to swap, and possibly another librarian - let me explain: If you feel up to it, you can chop the librarian and second
RW sargeant in half at the waist and then glue the librarian to the bike, this gets you a more mobile psyker (you can't cast most types of powers whilst in a Rhino). If this sounds beyond your current modelling skills then don't worry - just run the second bikes squad as a second bike squad.
~16000 Astra Militarum: 
~1200 | Imperial Knights: 
~2300 | Leagues of Votann: 
~1300 | Tyranids: 
~3400 | Stormcast Eternals: 
~5000 | Kruleboyz: 
~3500 | Lumineth Realm-Lords: 
~700
