Here's my thoughts:
Boyz in trukks looks and sounds fun, but in my experience it's lackluster, at best. Trukks have a fun habit of blowing up and killing half their payload, so they only work very well if you have like 4-6 trukks, and even then they only barely work. They are appropriate for armored, smaller units, especially meganobz, but little else.
For a new player, probably your first focus should be a warboss with a power klaw, painboy, and 40-60 boyz.
Expand from there to meganobz, tankbustas (and maybe a trukk or battlewagon for each), lootas, and perhaps killa kans.
Mek guns, especially lobbas and kustom mega kannons, are FANSTASTIC units - iron tough and dirt cheap, but they are the most expensive unit, $/point, in the ork codex, perhaps in the entire game. I'd recommend looking on youtube for DIY versions of mek guns - it's a fun projekt, and is going to cost far less to get a decent battery.
But my thought for a decent ork starter army is below. Above is unit, wargear, and points, below are thoughts on how best get them:
Warboss, power klaw, attack squig, bosspole, eavy armor - 109
(look on ebay, they don't sell a basic warboss for some dumb reason. Look for assault on black reach (
AoBR) or stormclaw warboss).
painboy - 50 - (buy in store)
3x squads of 20 boyz, each with a nob, power klaw, bosspole, and 2x big shootas - 490
(I prefer choppas over shootas, and am not a big fan of 'eavy armor, but EA would be a good idea to make your initial limited collection tougher and fill up some points.)
(Look on ebay for good deals, especially for
AoBR boyz).
15 tankbustas with nob, bosspole - 210
(official tankbustas are obscenely priced, just give a standard boy a rokkit and you're good to go.)
3x meganobz in a trukk with reinforced ram (ALWAYS take reinforced ram) - 155
(meganobz box is a good deal, trukk box is good too)
5x lobbas with ammo runts - 105
(make your own:
https://www.youtube.com/watch?v=8dBiM6sgL4A )
3 killa kans with grotzookas - 165
(buy box or look for stomclaw ones on ebay)
12x lootas - 168
(box or ebay)
1452 points total
If you're looking to just start, I'd probably recommend a warboss, 20 boyz, and 3 killa kans for fun, and expand from there.