PKM- you can buy everything you need brand new. No need to go hunting around for it.
I use a 4x5 glass carrier and masks I made myself for smaller sizes that simply lay over the carrier. I am not aware of a specific glass carrier for medium format. Omega made a glass carrier for their older enlargers that should fit, but I find it much simpler to just use the 4x5 one. The 4550 has mixing boxes for 4x5, medium format (don't know specifically what size it is) and 35mm, which has a small condenser in it. The prints I get from 35mm negs are incredibly sharp, more than likely due to the hybrid diffusion/condenser arrangement. They are sharper than what I used to get from a medium format Saunders VCCE that was purely diffusion. It is in effect a similar system to the Leitz enlargers- large light source, one condenser.
The 4550 can be mounted on a wall, so you can make prints as large as you wish. Wall mounting is the most stable method if you are concerned about alignment. The 4550 comes aligned out of the box, head matched to the column, but you may need to tweak it a little. Mine was used when I bought it and was off by about 2mm on the Versalign target. I added a tiny shim to the column, I believe it was a piece of paper. Once it is aligned, it stays that way. I still check it occasionally, but haven't had to touch it in a couple of years now. How many enlargers can you say that about?
I don't have a compensating timer because the 4550 uses a tungsten lamp. I don't think you really need a compensating timer for it. I use a Gralab 645 and my brain. It works well (the Gralab, the efficacy of my brain is debatable.)
You can still buy the Durst brand new as well. They are rock solid and have more flexibility than the 4550. If I ever had an opportunity to buy one that had the point light source head I would do it just to print 35mm. http://www.durst-pro-usa.com/L1200.php