Going to go with Golem's SPECIAL advice (4/6/7/4/8/5/6) except making STR 5 and lowering INT to7, so I don't waste an Intense Training perk in bringing STR up by 1.
In that case you don''t need to take Weapons Handling, but you will lose about 15 skill points over the course of the game. You'll still have plenty though.
Also going with Valkebus' suggetsed perk list except again I'll move the actual Cowboy perk up to lvl 8 as I think I'll need it with all the comments about high DT enemies.
You aren't likely to have many DT issues at level 8. Just don't go north of Sloan until you're a decent level and have powerful guns, and avoid getting close and personal with Giant Radscorpions. Smaller scorps are ok. If you must attack Legion guys in decent armor before you get at least a .44, aim for the head. Putting off Cowboy until mid- or even late-game isn't going to slow you down much.
Try to bring up your Lockpicking to 75 ASAP so you could get the unique .357 Lucky in Primm. It's the best early-game gun you can find.
A Hunting Revolver is found in REPCONN Test site. Won't have much ammo to go with it though.
Nearest place to get scopes for a Trail Carbine or a .44 Magnum revolver is 188. If they or the Gun Runners happen to not have any scopes, you will not have a sniping tool until their inventory replenishes except for the above mentioned Hunting Revolver with its scarce ammo.
As for companions going to try solo as much as I can, hence picking Sneak as a tagged skill although I can see I will need help at some point. If so I will usually go with Boone for his sniping skills and useful perk and probably ED-E. I've never used Rex or Arcade, am I missing something with these two?
I think Cass or Raul are better for your character than Boone- both because they fit the cowboy theme better and because of their perks. Cass has Shotgun Surgeon and can be REALLY helpful against precisely those enemies you have the biggest problem with- the high DT kind. Raul's maintenance perk helps negate the downside of Built to Destroy.
Arcade is pretty good, mostly because you can simply leave him with his default weapon and save ammo costs. Or you could give him a Multiplas rifle and Max Charge Ammo (-10DT), and he will do about as well as Cass does with a shotgun. His doctor coat is a nice fit if you think about it from the right angle; he can be Doc Holiday to your Wyatt Earp.
I fancy the Cowboy style hats and dusters but will miss the 1st recon beret. Only found a Sheriff's duster in Primm, is there anything better than that? I assume so.
Nope, I think Sheriff's duster and hat is what you should go with. Makes you the unofficial sheriff of the Mojave, badge and all.