THHN MUST be in some sort of conduit or raceway from box to box. You can use NM where it needs to be fished or run through framing. If all you are doing is stubbing up to W/M boxes the NM is fine for this all over.
I would place the boxes horizontally an inch or two off the floor with a short section of W/M down through the floor for protection.

NO, the boxes do not need to be 6" off the floor. They can be IN the floor if you like.

Thanks for responding so quickly. When you talk of "stubbing up to W/M boxes", do you mean just the wire through the floor, or can I use the NM in the conduits between each box as well?

If you can wire that easily using wiremold, it's not going to be much more effort to fish cable and install flush receptacles in the wall.

Have courage, man; plaster isn't fun, but it's not really THAT bad. I'm an old house fan and I've updated wiring on just about every old house I've lived in. I've used different methods for cutting in boxes. I started off using a jigsaw and coarse metalcutting blades purchased in bulk (they wear out very fast cutting plaster). That method is fast but it tends to rip the lath off the back of the plaster.

Currently I use, at various times and in various situations :

1. The jigsaw, when I'm in a hurry.

2. A Rotozip with a tile bit, slowed way down (otherwise it fills the house with smoke from the lath).

3. A keyhole saw that uses a short hacksaw blade; make sure it's set to cut on the backstroke so you pull the lath toward the plaster. This is the safest method when you have plaster that's somewhat decrepit.

One tip is to never cut out two lath. Always cut one lath, part of the one above it, and part of the one below it. This makes for a better box fit and a more stable wall. You may be able to find the lath with a mag studfinder, but often I have to drill some pilot holes.

You might like to try these boxes so you don't have to ask the plaster to support old work boxes. Then of course you have to locate the stud before cutting.

To be honest, I don't know squat about plaster walls - no idea what's behind the part that I see facing the room, if/where there's open space to fish wire through, how to find a stud without peppering the wall with holes, etc.

This isn't a house I plan on settling into forever, so I'm not too worried about looks. But do want to make sure that, since my primary tenants are students, that there's ample electrical service so they don't burn the place down with overloaded outlets and circuits. And of course I don't want to get stuck having to make a lot of costly, last-minute repairs when the time comes to sell.

Since you are primarily concerned with code & safety, make sure that you can legally do the electrical work. In our area a non-licensed person can do work on their single family residence (still requires getting a permit for the type work you are talking about)but they can not do that kind of electrical work on rental units they own or on multi-family dwellings.

Excellent point, Mike - thank you. Something like that never occured to me. I live here, it's my home (too), so I guess I tend to think that if it's "residential" then it applies to me and my house.

The more I'm learning, both in here and by book, the longer my list of questions to the powers-that-be gets. I've already done some basic wiring in the basement - added a string of lights - and have since found out that while it works and seems to be safe enough, it's not up to code (my dad's a great teacher, but apparently isn't up on code either). And so of course I'm nervous about asking any questions just yet 'til I've fixed that...don't want to get myself a red flag before I'm ready for them to come through looking for problems. Of course there's a lot of stuff PO's have done or had done that needs fixing as well, and I hope to get to all of it eventually, but I'm mostly concerned with the stuff I've done and plan to do. Ugh...I sure do hope I'm allowed to do stuff myself.