Personally Codex wise, I'd hold out a bit longer until you can look at the new Space Wolves Codex, I think that may be the best to suit your needs.
The big problem with doing a list like this is that the number of models you'll be using will be on a far smaller scale, as can be expected. Space Marines in general aren't really as versitile or cheap as Ork, with an average Pts comparission being 6 Orks for every 2 Space Marines give or take.
Personally I'm a long time Ork and Space Marine player, and my Space Marine army was built on my philosophy of Ork armies (the more the better).
The main advantage of Space Marines over Orks is better weapons to deal with high
AV vehicles, and superior
BS and
Sv scores. However you'll find that your units are more expensive and smaller; you can't move and shoot with most weapons; the bikers are worse; you only get a single Attack per model in general; and the army won't be as fun to play with no exploding stuff (excluding Plasma weapons) or wayward bomb squigs.
If you're still keen to go ahead, as I said, wait till you get a chance to look at the Space Wolves Codex.
Models wise, a lot of the old 1st and 2nd Edition Ork have a lot of models which could work well for your purposes as many used Imperial weapons and the model sizes are similar to Space Marines.
In regards to proxy models neither Dreds or Kans really fit the mold of a Space Marine Dreadnought, although Forge World has released a new Ork Dred with looks quite nasty. Most other models can be supstituted easily, Warbikers for Bikers, Meganobs for Terminators, 'Ard Boys for Tactical/Devastator Marines, 'Ard Boys with Rokkit Packs for Assault Marines. I think I'd actually save Power Armoured Orks for special units such as Command Squads, Honour Guard, Stern Guard and characters. Deffkoptas aren't the best fit for Land Speeders due to the weapon setups, but Warbuggies and Wartrakks are only for Attack Bikes as long as you add a Twin-linked gun somewhere on it.
I'll happily add more as you start to think of which army you are going to choose and what sort of things you want to do in more detail.
Cheers
Rowan/LSWSjr
Automatically Appended Next Post: Also thought I'd mention that for a Troops heavy Space Marine army, check my latest list on:
http://www.dakkadakka.com/dakkaforum/posts/list/258570.page
Cheers
Rowan/LSWSjr