I have a couple TM731s and they are a simple, sturdy and easy to use rig. They date from the late 1980s and you can expect that any used one you get will require replacement of the lithium backup battery.
If you can get the rig inexpensively, say around $200 for a clean unit in 8+ or 9- condition, I'd go for it.
The 721, which is very similar in appearance, but has an LED, rather than LCD display, is also a good rig, but should be about $30 cheaper.
The Kenwood 621 and 631 are the only dual-band 2 meter and 220 mobiles I am aware of, and they command a higher price of around $250 typical used in clean condition, because they are somewhat in demand for those wanting them for ARES and RACES.
In my humble opinion these older Kenwood rigs are made much better than the current generation of "disposable" mobiles. If a ten-year-old rig is in good working condition, I should expect few further problems with it. You may want to inquire to Cliff at www.avvid.com
as to the availability of parts to service them. He has repaired and aligned several older Kenwoods for our local RACES unit.
73 de KE4SKY
Virginia RACES State Training Officer