1. I assume you're talking about a 201 with a potted motor; my opinion would be the opposite, unless the 201 you're looking at has had the motor completely rewired and serviced. I've done a few potted motors and am not a big fan on servicing them, I'd much rather take apart a regular external motor; external motors for a 66 or 99 are much more easily replaced/upgraded. There are no upgraded motors available for a 201/potted motor that I am aware of.
2. I have found this site useful for explaining the different model variations
https://www.singersewinginfo.co.uk/
3. If the wiring is good then you can plug it into a regular outlet, but do not leave it plugged in unless you are using it. Unless it's been modified these machines will not have grounded or polarized plugs which means the electrical parts could be cooking while it sits there plugged in; if you're testing one always test it in a power strip with some type of circuit breaker built into it, in fact I would always use one in such a power strip even if the wiring is all fine; I have a small inverter than runs off my 18v power tool batteries and I use that for testing
4. The three you're mentioning, 99, 66, 201, would need a base or a table (although I think you might be able to get away with sitting a 201 directly on a surface, it's been a while since I've had one); the undersides of the machines are open and if you keep them lubricated as you should then you will be dripping oil on whatever is underneath the machine
For sewing the heavier materials you're talking about I would lean in the direction of a 66 or a 201. A 66 is heavy and a 201 is noticeably heavier than a 66.