Welcome to BP, @Ryan Miller!
I recently started getting into land flipping, so I don't have concrete answers to all your questions, but can pass along what I do know or have observed so far.
1. I think a lot of that depends on what area it's in. If other pieces of land nearby already have houses on them, then there's a fairly good chance the land you are looking at won't have problems perc'ing. I would also think that the bigger the lot or the more building site options there are on the lot, the more likely you'll be able to find at least one place that will work for a septic system. Some lots may not perc in lower areas, but if there is an elevation change, the higher areas may be ok.
2. For postcards, unless you are doing a mail merge version, you won't be able to add a custom field like a parcel number. In those cases, you can just match up the seller's name (and mailing address, if necessary) with the list you pulled and get the APN that way. Another option is to use mail merge when ordering postcards or letters so you can add in custom pieces of information like the APN. Taking this a bit further, I've also heard of people using a custom numbering method in their blind offers so they can easily identify when the letter was sent out and anything else they want. Something like having a small number in the upper or lower corner of the letter that might be coded like "mmddyycsapn", where mm=two-digit month of mailing, dd=day of mailing, yy=year of mailing, c=county, s=state, apn=assessor's parcel number. Or maybe just a couple of digits to indicate that it was the nth mailing of the year, or any number of other things that you might want to encode to make it easier to track which mailing it came from and which property it was for.
3. Not sure on this one. I haven't heard of any lawsuits related to the sale of vacant land, but I'm sure in our litigious society, they are out there somewhere. Perhaps if you do your own title search and end up missing something that the buyer finds out about later they might come after you for not selling it to them with clear title. To avoid this, you could pay a professional to do the title search and pay for title insurance, or sell with a Quit Claim Deed or Special Warranty Deed (instead of a general Warranty Deed). If you think you'll end up holding onto the land for awhile and it's a bigger or more expensive piece of land, you might consider getting insurance on it in the event that someone gets hurt while on your land.
4. Deeds can vary from state to state (and even county to county, I think). Some states/counties have specific margin requirements (so they have places to stamp for recording) or specific verbiage that must be included. Also, in some states, an attorney or title company must conduct the closing and draw up the deed. In states where you can do it yourself (and want to), one way is to look at the existing deed and use that as a template.
5. I haven't done any seller finance deals for land, but I have researched and dabbled in note investing. Personally, I would definitely recommend using a loan servicing company. It's not that expensive (maybe $25/mo), the cost can be passed off onto the buyer if you choose, and then you don't have to worry about the hassle of keeping the numbers straight and up-to-date, sending statements/invoices, etc. As long as the servicer you use is licensed in the state in which your property is located, it doesn't matter where the servicer is physically located. Most servicers I've seen are licensed in multiple states.
Hope that helps!
