Honestly, I'd avoid the devastators altogether. Far to expensive for what they do, and very fragile if you don't buy additional bolter marines for ablative wounds.
Since most 500pt games are played on a 2x2 board, mobility really shouldn't be much of an issue, so at this point level I'd focus on shooting and not worry about transports too much.
Here's a trial list that might be fun and fairly cheap to build.
HQ:
Librarian
no upgrades.
You can pick up the one from the Dark Vengance box on ebay for like $5 these days, with shipping
Troops:
10 Man Tactical Squad
Flamer, Missile Launcher, Sgt with Power Maul and combi-flamer
Get the
tac squad from Dark Vengance and a comander boxed set, do a few swaps, and you should be good to go if you don't mind proxying the
PC as a
ML.
These guys are running $20 or less on ebay right now or grab one of the sets from
AOBR for same price range. The commander box is about $15.
5 Man Scout Squad
3x Snipers, Heavy Bolter, Sgt with Sniper Riffle
Just bought a sniper squad on ebay for $15, with shipping, easy to find at that price if your patient.
Heavy:
TFC
This is the expensive one, but if you buy used you can find them in the $40 range.
500pts for about
$95 bucks or so, and it can be used as the foundation of a larger army if you take a shine to it.
If you don't have accesss to ebay for some reason, you can order Direct from
GW or just pick up the following kits for about $156 at your
FLGS
Commander Boxed Set, Librarian blister, Tactical Squad, Scout Sniper Squad, TFC
Tactics
For this point range you're most likely only going to see light armor (transports) and troops, maybe a few of the cheaper Heavy Support Choices
The Snipers can cover the
TFC at the back of the board (assuming 2x2) and work to pin down the enemy with sniper fire and difficult terrain (from
TFC).
On a roll of 6, Snipers ignore armor (rending) and can place shots, so kill off the Sgt along with special weapons in opposing squads. Unsaved wounds result in a pinning check
The
TFC gets four small templates per turn, can go
STR 12 with one of its blasts on a roll of a 6, and should wreck most transports. Additionally you have a
ML in the
tac squad if you need it.
It also decimates troops, especially
IG and Orcs, but in small games will work great against
MEQ as well.
Once destroyed the
TFC also gives you a free Techmarine with servo harness, a nice bonus at this points level.
The tactical squad combat squads into heavy weapons and counter charge, with the Librarian thrown in. (I tend to use Smite, Vortex of Doom (dangerous but fun as hell) and Force Dome).
The flamer and Combi-flamer can be used for wall of death or to soften a squad before the charge.
The Sgt is there to accept challenges, and since you don't need to worry about named characters, the regular initiative and high strength of the power maul should get you a wound on unnamed characters in most cases.
The maul will also work well on light armor, and will absolutely decimate anything with 4+ or worse save in
cc.
The Libby is there with the force sword if you need him to one shot something nasty with the force blade (
MC ect).
These games take under an hour, so mix and match powers and load outs to see what works for you.
Hope that helps