For me it would be the Raw TM1000 with the laminate thumbhole stock. If you contact Martin Rutherford of RAW in the states he can alter the length of pull for you etc. if you need to, and I have seen the stocks done in red laminate etc.
You can set the correct power for your pellet/accuracy. However, I have found that 11.3 ftlb is bob on for a .177 8.4 grn pellet.
It as solid as a very solid thing, typical american engineering with some imperial allen heads etc. Terry Doe said it is the slickest side lever he has used and I agree (I have had a Walther, EV2, MPR, Steyr etc).
The barrels are 16mm (like the Walthers) and are easily interchangeable. I have done a few mods with mine like fitting a splitter but that is just me.
I ran it over the chrono last week and it was +/- 3 ft/s maximum with the readout staying the same a number of times. This was with pellets strait out of the tin.
The block is bomb proof, I guess it is because he has designed it so it can cope with 30 ft/lb+ pressures with .22 and .25 calibres for the larger american/south african markets.
The TM1000 is a dedicated single shot target rifle with no facility for a magazine etc.
However, after saying all that I had a go with the FTP900 at the shooting show the other week and was mightily impressed, very accurate with not much to choose between the two rifles.
I just like the no frills solid approach to the TM1000.
I hope enjoy what ever you change to and make sure you hang onto that Walther for a while yet just in case