@Jaden Ghylin
1) Awesome that you've got a partner that owns MHPs. That's very helpful. To answer your question, even if you're selling homes for dirt cheap, mobile homes can take a while to sell. Hate to be a bearer of bad news, but generally speaking, lease-up in mobile home parks takes many years, not months, even if you're in a really hot area (some possible exceptions include parts of Florida and California). I would underwrite about five sales per year, which is a bit on the conservative side, so you're looking at about 13.5 years for just the 67 new homes to the property, not including the abandoned homes.
2) Good that it's not in a flood zone. The downside about private waste water treatment is just what you describe with the park requiring substantial cap-ex when they are not properly maintained. If you can get comfortable with this, you'll have a serious edge over other investors.
3) On cap rate, I've seen some sub-10 cap rates on properties that have needed hook ups to public sewer and on properties that have run out of water in the wells. This industry is getting a lot of attention from financial funds which has definitely drive cap rates down. I'm sure you have a target return that you need to meet that is driving a range of price you are willing to pay. From what it sounds like, hopefully you've underwritten this conservatively and have an additional expense contingency priced into your model.
4) Looks like you have done a good amount of due diligence. If you find that your numbers work, you're comfortable with its physical condition, you've got the capital and you have the patience to see this play out then you might just be in a good position to go through with this deal.
Best of luck. Keep us posted. I'd love to hear how it goes.