I have almost all of the M series primes, but very little experience with other series. I started my travel into legacy lenses with M's due to price. If you like to get the cheapest possible test set, I'd suggest M28/2.8 or 3.5, M50/1.7, M135/3.5 and M200/4. All are below 50€ lenses and produce nice rendering. Fifty is a special case and as good as anything short of new DFA*50/1.4, but others are not the sharpest in series, but still good enough for internet use.
If you are willing to put a little bit more money into the set look into M35/2.8, M40/2.8, M50/4 macro, M120/2.8 and M150/3.5. These should be around 100€ each. M120 and M150 are the secret lenses and sold well below what they are worth, as they are the best tele lenses in the series. Way better than M100 or M135. Hard to find thought. Also M35/2.8 has excellent value. And don't forget that M50/1.7
Other good options from M's, M35/2, M85/2 & M*300/4, are in a price group 200-350€. These might be worth the money, if you decide to stick in manual world, but I would start with cheaper ones.
I've shot Single In Challenge with all my M primes in APSC format and I'm currently doing them in FF. My skills have improved during the process, so lenses shot in the beginning are in disadvantage, but still it should show what the lenses are capeable to:
Ismo Heiramo?s albums | Flickr
My current favorites, AKA the group I choose travel lenses from, are: F17-28 FE, M35/2, M40/2.8, M50/1.7, M50/4, M85/2, M100/4, M120/2.8, M150/3.5, Tak 200/3.5 and M*300/4. I have added two lenses to cover the weaknesses of M series and can recommend both.