I bought a 6 gallon pot on amazon, no built in thermometer, but you don't need one anyway, a standard brewing thermometer is fine.
One problem I've encountered is my pot wants to "rock" on my smooth top electric range. If I replace this pot I'd look for one that doesn't have a totally flat bottom.
I started doing 3 gallon batches but increased it about 4. I also have a 16 qt cheap walmart pot that I'll also use to help get everything boiling faster with less chance of boilover. After 30 mins or so the volume is reduced, and I'll dump the smaller pot into the big one. The small pot is also useful to cook pasta.
I started with BIAB, but controlling the mash temp was a hassle, so I bought a $20 round cooler (5 gal) and do Bag in a cooler for the mash.
After the boil, I set the brew pot out at night to chill and siphon to the brew bucket in the morning. The setup works pretty good and doesn't take much space.