I have a sort of hybrid Saunders 4x5 enlarger. I have the 4550XL head, but with a mid-sized column and the smallest baseboard made for the 4x5 enlarger. I found that in printing up to 20x24 on the baseboard I need the 135 mm lens at almost the full column height. You'll need the tallest column made if you want to use the 150 mm lens when printing big on the baseboard.
As for the lamp power, having the 250W lamp is nice when printing big (especially big, slow Ilfochrome), but a positive hinderance when printing B&W 8x10's. There is an attenuator (a perforated metal disc that can be placed in the light path to kill about 3/4ths of the light) that helps a lot when working with sensitive materials, but I forget to switch it in sometimes.
I too have had some light leaks around the fam (fixed by realigning a metal baffle) and some around the film carrier, but have found that even with light-colored walls the diffuse light striking the paper during exposure isn't a porblem (and might even help a bit when printing slides by bringing shadow densitied up a bit). I tried the velvet skirt with a 6x7 Sauders enlarger and found the dust problem greater than the light problem.
The fan is noisy, but pretty much vibration-free. I haven't tested it, though. I can feel vibration in the housing, especially by the lens when it is extended very far, so it may contribute to a bit of unsharpness when the head is up high. I have looked at grain with a grain magnifier and can't see any vibration, but maybe that isn't an appropriate test.