I still think you are getting the short end of the deal.
- Just what functionality do you get from the Durst that you do not already have with your D5?
- Your Omega can go up to 4x5 film, same as the L1200. So there is no gain on the film format.
- Is the chassis of the D5 the long column XL or the standard column? If you have the XL, I am pretty sure that the XL is longer/taller than the L1200.
- Durst specific parts will be hard to find and EXPENSIVE when you do find them.

Did you make a list of everything that you will need to repair/replace or get to bring the L1200 up to operational condition?
- FEMONEG or BIMANEG universal negative carrier ($105 + shipping)
- FEMOMASK or BINEMA negative masks for each film format, to go into the carrier ($50-100 each)
- LAPLA lens boards (50mm+ lenses) ($35 + shipping for 39mm) any size other than 39mm thread will be more expensive (42,50,53mm).
- the missing power supply
- replacing broken parts ???

Go onto eBay to find the parts you need and price them.
Having been through this myself, finding some of these parts will take a LONG time.
Example the negative masks for the L1000 so rarely come up that I will have to have them fabricated from Omega D5 negative carriers.
I do not know if the negative carrier for the L1200 can use a modified D5 negative carrier as a mask. If not, you are stuck with the harder to find Durst negative masks.
My seat of the pants estimate of the power supply and the negative carrier+masks+lens boards for 3 film formats (w/o knowing what parts you really need) is around $900. The cost of the power supply and shipping for all the parts was a wild guess.
And if you need repair parts... The only way to get some of those parts is from people dismantling L1200s and selling the parts.
You know what parts you will need, and what shipping to you might be, so you can do a better cost estimate than me.

From a purely financial point of view, based on my rough cost estimate, it looks like a bad deal for little or no increase in functionality over your D5.
Is it worth it for other reasons? Only you can make that decision.

What is his straight purchase price, no trade?
Because, it seems that you are buying an enlarger that needs a lot of work for more money that it is worth, and you still have to put in a lot of money to get it operational.
You could come out ahead by buying the L1200 for $100, and selling your D5 for $400 or whatever you could get for it. And the D5 should sell for more than the L1200 in the condition it is in.

gud luk in your decision
and more gud luk in restoring the L1200 if you get it.