I did very well with a Dwarven grenadier. I used a boomerang as a backup weapon for when I didn't want to use up my grenades, but it's quite easy to make a lot of molotov cocktails so you can use those quite frequently. Also later on you'll get useful things like stun grenades. You can hit an enemy with several bombs and then right before your turn ends, hit him with a stun grenade, and he'll miss his next turn. Then keep bombing him. Also, rather unrealistically, bombs will not hurt you or your allies, so you can throw one at an enemy who's right in front of you and it will push them back.
For that build, you need throwing skill, decent Dex and Perception, and the Explosives tech college. You'll want to boost your Intelligence too so you can gain skill in the Explosives college which will unlock more powerful bombs.
You'll also need to spend some time searching garbage cans for junk... all you need is rags and fuel to make molotov cocktails, and you'll find lots of rags in garbage cans around the cities. You can also check junk dealers for parts but you'll have to pay for them.
I think a grenadier might be easier than a gunman, if you're worried about difficulty. But really you should just pick whichever build sounds more fun.