If the investment was held for at least 10 years, the taxpayer is exempt from capital gains tax on the sale.

Reg. 1.266-1(b)(1)(i) allows the taxpayer to capitalize the following items: (a) annual property taxes, (b) mortgage interest [which would otherwise be subject to the IRC Sec. 163(d) limit on investment interest expense]), and (c) other carrying charges such as the cost of mowing and pesticide applications (which would otherwise be subject to the 2%-of-AGI floor for miscellaneous itemized deductions before 2018, thus starting 2018, these are not deductible)

@Frank PerazaDepends on who/what you quitclaim it to...Quitclaiming to a business entity in which you have an ownership interest, generally speaking no.Quitclaiming to your child or friend in non arms-length manner in which fair market compensation or consideration isn't received in return, generally speaking you'd have gift tax exposure.Lifetime exemption comes into play if the dollar value of gift exceeds the annual exemption per taxpayer per donee.

After several years the room-mates left & she needed to downsize into a subsidized (paid by taxpayer) apartment.

@Mary RobertsThere is case-law that provides guidance on when a taxpayer is classified as a dealer or an investor.As @Jay Hinrichs pointed out; it depends on the facts and circumstances of the taxpayer such as intent, holding period, frequency of transactions, etcFrequency of transactions - the more transactions you do in a year; suggests that you are a dealerIntent - if you intend to buy and quickly sell, suggests that you are a dealerHolding period - Shorter holding periods among all your sales suggests that you are a dealer

If you're setting up a partnership then that partnership is going to be the taxpayer for the property. 

But, If you understand this, it will answer your question:Estimated Tax Payments must be:Lesser of 100% of the tax shown on the taxpayer's return for the preceding year or (110% if 2017 AGI was more than150k)90% of his tax for the current year.Assuming you had no other income and you didn't owe any tax from the flips last year-2017, you are not required to make payments this year-2018.

If you understand this, it will answer your question:Estimated Tax Payments must be:Lesser of 100% of the tax shown on the taxpayer's return for the preceding year or (110% if 2017 AGI was more than150k)90% of his tax for the current year.Since you didn't owe any tax from the flips last year, you are not required to make payments this year. 

Is there a property tax payment coming due on this place?

@Michael M.Cost is important but you want to vet the professional that you are working with.With that said - the best way to get a good projection of what the professional fees you would pay to a tax accountant/advisor is to interview a couple and get some price quotes.Your tax profile is more complex than the average taxpayer as you have properties scattered across different states.Are any of your properties inside of an LLC?