An overlocker would be an absolute boon with stretch materials although you will sometimes need a sewing machine for some tasks.
If buying or space for two machines isn't a practical option, I'd strongly advise you look at the Pfaffs, specifically the ones with IDT (dual feed) which will resolve the feed issues that many other machines have sewing stretch. Pfaff invented the system and includes it in a wide range of its models. The patent ran out I believe so Janome and Bernina also have a similar feed system now but only on the very expensive models within their range.
A walking foot can often improve the feed issue associated with stretch fabrics but you can't use one with a wide variety of other feet and it's so wide that visibility and manoeuvrability can be tricky.
If you're only limitation is the budget, you can check out used Pfaffs as they have been using the IDT for decades hence their older machines are great buys (some say better than the new ones since they were gobbled up into the giant SVP conglomerate). This may mean you can manage a small overlocker too which would be a great setup. Overlockers are very sturdy mechanical machines (only a very few have any computerisation) so they are have great potential as used machines too.
Good luck!