I have a TD145 mkI and a TD125 Mk1. The TD125 is completely manual. I have an rb300 on it. The td145 has the stock thorens arm. I like the sound of both, altough the TD125 is better.
I bought both thorens on ebay. There are usually a couple of each type active. TD145's go for 80-150 depeding on condition and the usual vagaries of ebay. TD126's are more expensive, especially with the later versions. There was a nice MKIII lately that went for over $200. I stopped watching it because I felt tempted. I have been toying with the idea of a dc motor replacement in one of thorens, so why not just get one with a DC motor?
Any semi-automatic Thorens uses a thorens arm. My opinion is to go for a later arm. The original tonearm on the 150 isn't very good. The TD160/TD145 arm, TP16 Mk1, is better but has a fairly high effective mass of 16.5 g. I think that is too high for high compliance moving magnet cartridges that should probably be used with it. So I would look for a later arm with a lower effective mass.
The TD126 is mostly the same as the TD125, but with auto-stop. I haven't heard it, but the TD126 MKIII is considered the best because it has a DC motor. Earlier Mks and the TD160, 145, 150, 125 are all AC motors.
Make sure you ask if the auto-stop works. There is a pretty complex electronics circuit that controls the auto-stop. While you are at it, ask if the motor spindle is bent, if the dust cover still works, and if there is any rust anywhere or if there is any hum. Make sure the end weight is included, and the 45 rpm adapter.
Bent motor spindles are common on Thorens from improper shipping. Always remove the platter for shipping.
Good luck, the hunt is half the fun!