I am in need of assistance in a comparison of the Beeman 400 and the Walther LGR. It is my understanding that the Beeman is actually the Diana 75, is this correct? I have no knowledge of the Walther other than it was/is a target rifle. Your thoughts please.
Yes, the "Beeman 400" is an early version of the Diana model 75. It is a spring-piston gun with sidelever cocking, using a complex double-piston mechanism. It is easy to cock, loads via a sliding breech, and will shoot light match ammo at a little over 600 fps when healthy.
The LGR is a somewhat later design, and is a single-stroke pneumatic. It loads via a flip-up port. It is harder to cock and less powerful than the 75, usually around a 550 fps gun.
Both guns have superb workmanship, accuracy, sights, and trigger action. FWIW, most would give the slight edge to Walther in all 4 categories. Both are great guns that will outshoot 99.9% of us though, and are quite utterly recoilless in action. I hope you are in a position to try them both and see which you prefer.
The LGR was the more successful competition piece in its day. It really replaced the Feinwerkbau 300 series as the top match air rifle in the world in the late 70's/early 80's. The 75 was very popular, sold in large numbers to clubs, etc., and did well at most levels of competition. But it never really cracked the medals at the international/olympic level.
It's very likely that both guns will need seals if they don't have recent rebuilds. Be sure to ask about that.