For what it's worth, my experience and advice is almost the same as Jane's. I started with FTDNA and the whole experience was good and inexpensive, then later also tested with Ancestry, which ends up costing a lot more. You can test with Ancestry and then upload to FTDNA, but the test isn't quite as good, and it doesn't cost much to do the separate test with FTDNA.
One thing to think of is the number of people from your region of interest who are likely to have tested with each company. I researched this before I started, and found that while Ancestry's database is by far the biggest, it was much more USA centric than FTDNA, and I calculated that there would actually be more matches in Australia (where I live) with FTDNA. And that proved to be the case - about twice as many matches at 4th cousin level. Not sure if it would be the same now (that was 2+ years ago) and in Europe where you are interested, but it's worth checking out. Also, is My Heritage more European based? I don't know.
But if you have enough cash, I'd suggest testing with both FTDNA and Ancestry, or else FTDNA and My heritage if they are mostly European, and then upload to FTDNA.
I also agree with getting your mother tested, because it helps eliminate many matches as not being from your father's side. I ended up arranging for an aunt, a cousin and a distant cousin all to test, and that really helped. But of course money doesn't grow on trees!
It may encourage you to know that after several years of hard and frustrating work, one of my mysteries was resolved very quickly when a good match was obtained between someone I didn't know and my aunt, and the mystery all fell into place. But that was after about 8-10 years of paper searching and 2 years of DNA with 5 tests. So it is worth doing and being patient!
It may be that someone will just pop out of the woodwork and solve your problem, but it may be that it doesn't happen that way, which means you have the hard task of trying to connect matches to you via their trees and yours., via paper records. This can be hard work, and will likely require several trails that don't lead anywhere before you hit on one that does. So you want to choose the best possibilities. There are several ways to do this.
(1) If your mum tests, chose people who don't match with her.
(2) Use the "in common with" tool in FTDNA and the equivalent in Ancestry to find who is in common with you best match, and again with your second best match, etc, then choose the person who has quite a few matches in common, because you may find some common information there. FTDNA's chromosome browser is very useful here too.
(3) Obviously choose people who have good family trees, and/or who are cooperative and answer your emails.
Hop that helps.