I'd get the 50 regardless of the price on the 40........or you could use an 80 mm. Don't know what you're enlarging that you need a 40 mm. If it's 135, then use an 80 or a 105 which will work on a Seipla/Seitopla and be done with it. L
That's scrunching the bellows pretty hard, and might disturb your level. I can't recommend it, though I have often used a basic 50 for modest enlargements just fine. I prefer something longer, at least 75.
Yeah, I checked, it gets pretty compressed. I don't think the 40mm would work for bigger enlargements. It's hard to say no to a good deal, but I'll have to pass.