Holy crap that Arbites collection went for $378.
For your needs:
Keep in mind,
Sisters are a fun army to play but you aren't buying into a "fair" army to play. You'll be losing a lot especially against mechanized.
Your gear options you described are GREAT for anti-infantry. 4 storm bolts is 8 shots, 16 double tap. Using the acts of faith and shooting twice in your turn for a squad with 4 (dominions) + the rest of the squad double tapping twice. Situational I know, but on the defense it is insanely good. Or offense if you can close distance. Just hope you have cover + can gun down melee before they melee you.
Closing the distance won't be hard - 3 immolators means 3, 5 sister squads can get in while you get into melta-range. It also means your 5 sisters have to get out and not die horribly... so they will need to be carefully used in this manner.
3 Immolators are great. Make sure you have all the bits or they come with melta, I can't re-emphasize this enough.
$200 is pushing it. $80 a squad new, and on ebay right now i see several auctions at ~$40-50 for 10 or so used (and easy to strip - all metal.)
So to get your 30 bodies on ebay you'll spend at least $120-150 or so. Flamers and storm bolters are not rare at all.
The immolators go for ~$60 depending on how well they are painted etc. $40 NIB but they are out of stock so ebay prices are high. So you get 3 of those? That's another ~$120 if you got NIB from GW. ~$150 if you got from ebay, or more.
TLDR: Your sisters are worth $360 NIB - and on ebay probably about $300, maybe much less if you find cheaper immolators.
Would I pay $200 for that? Depends heavily on how the immolators are, what the paint job is like, and so forth. It isn't a rip-off (as much as I can say that about over priced sister stuff) - but you could find better deals if you really tried.
I say go for it, if you like it you can enjoy it, if not, you can sell on ebay and get most your $ back.