Can I ask for credit up to the amount of replacing the hvac system which I doubt the seller will accept or just pull out of the contract?

It costs so much to sell a property and buy another one, and is such a hassle, that if I were in this situation I'd strongly consider just paying the special assessment and keeping a property that's otherwise great.

About 50% of investors buying a tax certificate think they’re buying the real property2.

Getting to an asset manager to buy a note is not impossible, but definitely not easy either.

Do you know what year the roof was last replaced?

It shouldn't be normal wear and tear, however, you can't charge them the full amount to replace them. 

There apparently was a guy that was very active on BP with over 4,000 posts, regarding buying subject to properly, but has either been booted off or canceled his account.

I always felt families buy homes and retirees buy homes. 

and who did they replace him with  Guest hosts ??

For example, the tenant may tell me they can't afford to pay for a $2,500 replacement door, so I'll negotiate a payment plan (my PM Agreement authorizes me to negotiate on the owner's behalf) and then I will update the owner on that change.