The only real Q45 is the 90-96 G50 chassis, IMO. The later years were just rebadged Nissan Cimas but my views are a bit biased.
I have a 94 that is turbocharged and a 95 that is mostly stock. The engines are relatively trouble-free and will last for a long time if properly maintained.
My turbo Q is running approx. 8 psi and it makes around 450 hp. This is with a 240k mile stock motor including the injectors. You will be hard pressed to find a stronger engine than the VH45DE for such a low price. They are as strong as the early 1UZs and some would even say the VH surpasses the 1UZ.
The early LS400 and the first gen Q45 are special cars with different ways of doing things. I've always considered the LS to be a luxury car with sporting characteristics. The original Q45, on the other hand, is a sports sedan that has luxury features.