What I want:
MAX dmg, High/VH ASI, VH CTR on guns, and High+ fire resist
What I have:
Shadowsun Slicker
Elite Trueshot mod
Swiftstrike
What I can do:
1) Buy perfect mask of seerus (from a friend) for 4.8kCE and use another Elite Trueshot mod
2) Craft shadowsun stetson and use Elite Gun focus mod. Try and UV roll a fire UV
3) Buy a fire UVed gunslinger hat (High or MAX), craft it into shadowsun and use Elite gun focus mod
Which of these options is likely to cost me the least?
craft, but you don't really need to roll fire UV, you should be perfectly fine on FSC after playing there for a while unless you get bad connection. Or rather, save up to buy expansion pack from steam trade, that way you could get "free" new sword + bomb that you may play it sometimes. UV roll on something else that is more profitable for you.