I concur with both previous posters though my needs and uses can be slightly different from yours.
I currently have access to all of the machines mentioned, I own all of them except the DD40 - which I sold off to a colleague who is in the same workshop so the machine is still within reach.
If you tell us more about what you are building it might be easier to help you find your perfect match. Personally I really enjoy all the machines but I gave up the DD40 since it was the "least" useful machine for me.
If you build a lot of boxes or cabinets you could pair any two of the machines - except for the Domino XL700 - and you would do well with that pair.
A Lamello Zeta P2 pairs extremely well with the Domino DF500 as well as with the DD40.
The DD40 is the least forgiving of all the machines and this is both the blessing and the curse. Mess up a pair of holes and the box simply won't come together nicely. There is literally no play in the placement. Should you work with massive wood or slightly warped sheets this might be a real problem. If you pair the DD40 with a DF500 you can use the Domino for the stuff that needs a little lateral play to fit. This is how my colleague uses his DD40. He uses the Mafell a lot more than the DF500 building kitchens. But he has good use for both.
I am the opposite: I could not get by without the DF500 as it solves most everything for me "on the fly". I have used the DF500 since it came out on the market and I can attest to the range of use being a great deal larger than what it is designed for.
I pair my Domino DF500 with the Lamello Zeta P2 to great effect and sometimes in combination. I would not want to be without either machine and the Lamello has grown on me. Still if I were to list the machines in importance to me it would look like:
Domino DF500 general purpose, not too expensive fasteners.
Lamello Zeta P2 general purpose and glue ups, knock down assembly. Expensive fasteners.
Mafell DD40 kitchen cabinets and LR32 holery.
If you get one of the hole row making templates from Hettich or use the Festool LR32 you can bet by without the Mafell DD40 and pair up any other two machines of choice. Sure, the dowels for the DD40 are dirt cheap but the machine is very expensive.
I have left out the Domino XL700 in the above tirade on purpose. It is my least used machine but when I have needed it it has been _the_ machine for the task.
If you find yourself building some larger furniture the Domino XL700 suddenly makes more sense in combination with any other of the three: it makes a lot larger holes and can accommodate larger dowels that you can make yourself. And you can use it with the knock down fasteners they sell for it which work great. You can also use it to plunge larger starter holes (or complete groove holes) for doors that are to be assembled. Some of the larger builds I have made would have been a lot more tedious to do without the DF700. It has paid for itself on one or two builds and is a keeper regardless of what comes next.
For overall versatility I went with the DF500 and the Lamello Zeta P2.
I am keeping the DF700 even though I don't use it as often. It isn't as clumsy in use at it is to simply "dry swing".
If I was doing custom kitchens more often I might get the DD40 - but I might as well get a dowel boring machine. My colleague is getting a dowel boring machine to partner up with the Mafell DD40(G) though he was contemplating another Mafell since he switches between dowel sizes now and then and mainly does custom kitchens.
Out of all four the Mafell turned up to be the only one I can be without. Just like Jimbouk I decided I won't run LR32 holery per any standard length of run.
I run them as I need them and have or make templates for that.
The Lamello is really nice to work with and does a few things that makes it stand out for me when I am doing acoustic panels and frames. It literally saves me hours in glue up and makes it possible to work without a mess on site. Worth every penny for me though for some simple joinery or work the fastener components can make a shelf verrrrryyyy expensive when you start churning them out...