You're on the right track with your idea. I wouldn't recommend heavy armor for pistol/SMG characters though. I'll explain a little bit further down. Both weapons are considered light, so more dexterity you have, more attacks can you do per round.
This chart is dexxer his best friend:
http://www.underrail.com/wiki/index.php?title=DexterityAnother important stat is perception. Guns skill directly corresponds with it and both, your chance to hit and damage increase along with it (no law of diminishing return here). Idealy you will want to start with 10 in both perception and dexterity. The question weather you should focus on dexterity or perception on your level ups depends on your burst weapon choice:
Is it SMG then go for dexterity. If you mainly want to use the assault riffle as the burst weapon then raise perception as all your weapons profit from it, as compared to dexterity which doesnt help ARs at all and I personally prefer those over SMGs which sound great in theory, but in the field you'll start to realize that they are very weak against enemies with high damage threshold. They are ok with gun nut and expertise if used situationally. Not excellent but ok. Another argument against SMGs is, to be really effective with them you need 6 agi for the spec ops feat. You can't afford those here. Only good for a pure SMG build that swaps str with agility.
Commando feat gives you 1 free burst upon killing an enemy with burst, so better to use that with a strong weapon.
In any case that doesn't leave much room for other stats. 6 strength is minimum for the steadfast aim feat which I consider a must for pistol characters. You could actually raise strength to 8 and dump everything else left:
8 str
10 dex
3 agi
3 con
10 per
3 wil
3 int
However there is a problem with this. Heavy armors only make sense if you can reach 95% mechanical resistance cap, or at least be close to it. With 3 con you cant pick conditioning, so with ancient rathound leather tabi boots (+5%) and lifting belt (+10%) you need a heavy armor with at least 80% damage resistance. Possible, but not without a hefty armor penalty and you cant take armor sloping here. Penalty to dodge and evade will be not much of a problem, you're almost immune to mechanical damage at this point, but as a walking can you'll have very few movement points. Sure, your weapons are ranged but it is very limited. Quite so for pistols, even more for energy/chemical ones and if you want to perform execute, you'll need to be almost in melee range.
ARs are not that bad though but it would be waste to focus only on them and use (electro)pistol only if the enemy happens to be near enough.
To get back to the actual build. This is what I would recommend:
Like I mentioned in the beginning, heavy armor is not the best option here. Better idea would be to set strength to 6 and use tactical vests. I'm not very fond of riot gear for nimble characters. They can be very useful, especially the shield variant but you cant use that with ARs. Besides that, to get the 10% nimble bonus you need an armor with 15% armor penalty or less. Riot armor always has at least 20%.
So stick to light tactical vests made with reinforced fabric ballistic panel which favors resistance. Don't use armor plates. Going after high damage threshold is only really worth it with ballistics feat and kevlar balaclava/tabis. You cant pick that feat here and need the headgear slot for goggles. Conditioning + ancient rathound leather tabis + lifting belt + good vest will give you solid mechanical damage resistance.
Use
http://underrail.info.tm to decide how to spred your skills. Go full on guns, dodge and evade, few points in traps. Lockpicking and hacking up to effective 75 (including tools + Jackknife) and then just enough so you can open the locks (you previously failed) after next levelup, psychokinesis 25 for force field (very underestimated psi ability), metathermics 35 for cryostasis (together with electropistol you can keep 1 enemy stunlocked), for crafting you'll only really need mechanics + electronics + tailoring. the rest of crafting skills only if you can afford them (which you probably won't), initially up to 50 into throwing and additonal points only when you dont need them right now for crafting. Make sure to use junkyard suprise for temp int bonus. I usually dont aim for +2 (small chance) until I have hoarded lots of them but instead for +1 (much more likely to get). With +1 you'll have 4 int which will offset your -10% skill penalty you get with 3 int.
A major problem with this build is you use more than 2 different weapons. I always recommend amplified electroshock pistol in one slot. Seriously, that thing is so OP, with crits you often kill 3 enemies that are around your level. With so many burst feats, you also want a fast AR in the other slot, but you also shouldn't miss doing executes with a .44 cal pistol. The issue is you would have to waste full 50 ap points to swap a weapon from the backpack.
So one alternative idea for this build would be to replace either full auto or supressive fire with quick pockets for 25 ap weapon change instead of 50 ap (one extra utility slot is helpful too) and take it earlier in the game. That way you can start battles with a sniper from afar, with lots of grouped enemies burst them with an AR and for vs 1 or vs 2 a .44 cal. I haven't playtested this particular build yet but I'm sure it's not only very versatile but also fun to play. Carry weight limit might be a problem though.
If you're in for min/maxing, the way things work right now with amplified weapons it's best to stick to a max dex build with pistols. Still room for quick pockets and strategical gameplay with a sniper until the enemies are near enough for the pistols. Skillwise only difference is stealth instead of throwing. All in all something like this:
