Tough call. €1200 for an Ultra and MFC is a good deal, on the other hand, since you still plan to use an amp, €1400 for an all in one FX only unit that is more modern and has more modern features might be a good investment too.
Pro of the Ultra, you're going to bring along a 19" rack along anyway for the A3, so its not like you have to get one extra for the Ultra. It can go straight into a mixing desk if you ever desire so, as it got amp sims. It's €200 cheaper.
Con of the Ultra. It's a legacy product. It will never be updated again and your editing software might no longer work on new operating systems. It doesn't have some of the new cool features of the current Fractal modelers, like X/Y switching. It may or may not have scenes either. Those are features I've really come to like and use a lot. On the plus side. what you never had you'll never miss anyway. As its an old unit if it breaks down you're boned as you will have no warranty. Fractal can probably still fix it, but it will cost you. On the plus side Fractal gear seems to be well made though.
Pro of the FX-8, new top of the line unit, which latest features like X/Y switching, 8 scenes per presets (which means you have 8 sub presets for every preset). It will still receive new upgrades, updates, new FX being added and improved, the whole shebang. It's a stand alone unit. No need to bring along a big 19"rack with the processer stashed in it. You can run the A3 from it in the FX loop. As its new you'll have a few years warranty as well in case something happens.
Cons, FX only, so no going straight into a mixing desk if you ever desire or have too. You still have to bring along a small 19" rack for the A3 and run cables to and from it to the FX-8. And its €200 more expensive, so you're not saving that much stuff that you have to bring along for a show, plus 2 more cables for the A3 = more cable spaghetti on stage. Also I'm not sure how you can send MIDI orders via the FX-8 to the A3. If its possible, it probably is, somewhere buried deep in the manual, it doesn't appear to be a feature of Axe-edit as I don't have it on the Axe-FX and AX-8. If its not possible you'd have to switch presets on the A3 manually. But I assume you only use it for Mysterious Ways, as most guitarists seem to do who still use the A3 together with an Axe-FX, so this may not be such a big problem after all.
Anyone feel free to add.
Edit: I just remembered some stuff. With the MFC you can give the A3 MIDI commands for sure. The major con is that while the regular editors for all the Axe-FX, old and new, AX-8 and FX-8 is free to download and use, you have to pay $30 to download and unlock the editor for the MFC. Which is a kind of scam in my opinion. I can understand that additional DLC for the gear you're selling might be costing something. New stuff you're adding for a product that is already sold costs money, it has to be recouped at some point. But its optional content. You shouldn't have to get it. Basic editors you HAVE to get. And charging for them is wrong. Then again, the guy whose selling the MFC to you might already have both the downloaded MFC editor and the registration key. Be sure to ask for them if he hasn't mentioned them yet.