I use a P45+ with both the latest 40mm V series lens (the IF lens) and also the 38mm Biogon. Here's some thoughts,
1. The 40mm IF is significantly sharper than previous Hasselblad 40mm lenses, this is true at all apertures and right across the frame (although the very largest benefits are out towards the edges at medium to wide apertures). It's even visibly sharper than the 38mm Biogon.
2. The key downside of the 40mm IF (besides the jaw dropping price) is the dreadful distortion. Two points about this distortion. First, it's worse than any other Hasselblad lens (current or past) except for the 30mm fisheye. Second, it's not a simple distortion, so it's actually difficult to fully correct in Photoshop, because I mainly take architectural shots this can be a real problem.
3. The 38mm Biogon SWC/M is noticeably sharper than the Hasselblad 40mm lenses prior to the IF lens, furthermore distortion is virtually zero. It does however vignette very badly, although this is normally very simple to correct in post-processing.
4. There was one final version of the 38mm, just before the lens was finally deleted, I think it was called the 905 and it involved a new optical design. This change was forced on Zeiss/Hasselblad as one specific optical glass type was removed from the supplier's stock list because it contained undesirable elements like lead and arsenic. The new design gave some tiny improvements out at the edges at open apertures, but the central performance dropped by rather too much to make it a desirable trade-off. Avoid the 905.
5. But also be careful if you're getting a 903, because some of the earlier versions had a shutter problem which could cause the shutter to brush against the optics causing micro-scratching. Unless you're experienced with this issue you're unlikely to spot it, but if you subsequently try to sell the camera to a dealer they'll certainly spot it and reflect it in their offer price! The simplest way to avoid the problem is to just make sure that you don't buy any 903 with the level bubble built into the body. This is a crude precaution as actually there was one year of production with both the new (non scratching) shutter and a level bubble in the body, I'd argue that this is the most desirable 903 of them all, but to keep it simple just say "no" to body bubbles!
6. I find the later 903 viewfinders are pretty easy to use with a 37mm x 49mm digital back. Just to maintain the bubble theme, you should look for a viewfinder where the bubble's visible during composition, then you know you've got a later one! There's vertical/horizontal composition lines in the viewfinder that allow you to compose with surprising accuracy. However, for architectural applications I normally use ground glass focusing, which is painfully slow and the ground glass costs a king's ransom. Unless you're at infinity or f22 you'll find focusing innacuracies will really take the edge off digital resolution, and sadly the range focusing of the 903 can never really be that accurate for demanding applications.