It is very common, at least in CO for all cash offers to complete due diligence in 2 weeks. One of the major benefits for a seller with an all cash offer is less contingencies. Because there is no loan, it is typical, at least here, to waive the appraisal. Personally, I would not call this a red flag.
The red flags that I see, is the location. Being a small town, I would definitely be concerned about the town dyeing. What is the major economic driver in the town now?
I am going to assume that you are offering cash then plan to do a cash out loan. If I am correct, have you done research on a commercial loan and have a good idea of the interest rate/amortization schedule and know how it will play with your numbers?
I would also ensure to walk thru every unit. It's not unusual to be shown a couple of the best units to be blow away later by the rest of the units.
It seems odd to me that only 1 unit needs re-wiring. why is that? I would expect if one needs it then most or the entire building needs it.
Get on regional and see what permits have been pulled for the property. When was the roof redone, water heaters, furnaces replaced. You should also be able to see any electrical work that has been done.
Finally, request at least 2 years of rent histories and copies of the leases. Sometimes a seller will get rents up to raise the CAP rate to sell. That's not a problem, but sometimes it's not sustainable for that area/building without massive turnovers. As you know turnovers are very expensive. It sounds like you have researched that areas rents, but neighborhood to neighborhood sustainability changes.
If you are very clear about the work needed and the local economy, I would say the number are fantastic! I hope it works out. If all is good, I'm jelly. 😉