@Adam Marx what are you intentions with the property? Is the ARV to flip or to buy and hold? You said 'I'd put ARV safely at 225-275'. That is far to big of a spread, you need to be much more accurate especially if you are flipping it.
Some things to consider if you are flipping. Maybe pay for an appraisal up front or if you are financing, the lender will do one and if it doesn't appraise, you can walk away with no consequences other than your time. Are you working with a Realtor® or a contractor, both can likely solve your problem as the agent will have access to more data and knowledge of how to run comps, the contractor will be able to give you a better idea of repair costs. Then tell the agent to run comps for ARV based on updates you do.
And you also said you can't afford to lose $20-30k, the most seasoned flippers/investors lose money all the time, and ultimately that is part of your education costs, hopefully you don't make any costly mistakes, but you have to run your numbers well enough to make sure there is some 'ooops' fund to help shield you from that.
As for comps, typically, appraisers want them to be in the same neighborhood, within 15% same square footage, same # beds/baths and age. If they can't find those things, they will go out a little further (start at 1/2 mile, then mile and so on) Then they will make adjustments to bed/baths/sqft. Say yours is a 2 bed/1 bath all other comps are 3/2's. They would subtract the value of a bed and a bath, maybe $7500 for a bedroom and $5k for the bath and then factor the same for other things like pool/no pool, 1/4 acre vs 1/2 acre and so on. So you can do the same thing, but you have to know what a bedroom is 'worth' in that area. Some areas a bedroom is $5,000, some $10,000.
Something else you could do since the property is off market, is negotiate with the seller to bring them in the deal. I'll pay you 'X' and also X% of the profit when I sell. So the better you do they make more too. But if your ARV is way off or you finish the flip 5 months from now and the market has shifted and you don't make much, neither do they. Lots of other ways to structure those too with the seller, thats just one example.
Best of luck, keep us posted!