Great answer by "SJC" above.
I use an MTH 4000. I paid about $400 for it a year or two after they first came out (maybe around year 2000). I saw a used one in a White Plains, NY train store about one month ago for substantially less (my recollection is that it was about $150 to $200). Might be a good deal if you can get a good technician to examine it to make sure it is in good condition.
It's nice to buy new and get the warranty.
In addition to the MTH 4000 to run my 2 separately blocked main lines, I also use 2 postwar ZWs to run sidings, switches, and accessories. They are quite economical used and in good condition (maybe $100 to $150). For the serious and knowledgeable adult hobbyist, I think the post war ZWs are fine. One drawback is they don't have the bell function, I agree they do warm up after 10 or 15 minutes running trains. They are work horses with good power, easily fixed by qualified technicians and there are plenty of available parts.
