Saw my first TRGS at the fun shoot last year. For a factory tic tac rifle, it was awesome. Shot extremely well even under some pretty long strings. Was the only rifle that showed up that tried and completed the one mile challenge. Think it took 6 shots to go from 725yds to the mile, but we didn't see one land. Very impressive.
Now saying all that, there are alternatives IF you want to build something and am not fixated on the Lapua case itself. A 338 RUM, 338-300RUM/Mystic, and 338 STW/340Wby improved would do very similar things (if not more) with way less cost in the brass. The 338 RUM even has factory dies (well so does the Lapua now).
All of the above will fit on a standard mag bolt head so any action can be used. With a Howa, rem700 or Savage (if you can find them now), they will also all shoot from a mag. Not a dmag arguably but I shoot my LR plinkers single shot so a mag really doesn't matter anyways.
A Stevens LA in 300WM or 7RM would make an ideal donor action. Bet you never heard that before
The TRGS stock is not ideal for bag shooting. This is a field stock/bipod use. If more target/bench then hunting, not ideal but certainly functional and lots of adjustments. I found the grip much more comfy then an AI but your mileage may vary.
For less money, you can build up a big heavy barreled bench gun or a field rifle.
Think of it this way. You can buy a BMW/Porsche and get a certain level of performance. You can tune a Camaro/Mustang/Honda and get very similar levels of performance for way less money.
At the end of the day, which do you want in the driveway...
Jerry