Get an old-ish Mac Mini (early 2010s models are still fairly decent by today's standards, definitely good enough for a secondary development machine / OSX testing). It won't have an SSD, but you can always install one if that's critical.
With that being said, upgrading them to an SSD probably won't improve much. The disk drive isn't the bottleneck; my Windows machine has the same speed and amount of RAM, and the same speed (but larger capacity) HDD as my Mac Mini. Major difference - the Mac Mini has a dual-core 3rd-gen i5; whereas my Windows machine has a quad-core 6th-gen i7 - the actual GHz difference is almost negligable, but it's a much newer (and more cores) CPU. (The Windows machine also has an extremely superior GPU (GTX 960 vs Intel HD 4000), but I doubt that's relevant.) Both have been recently reformatted, so the issue isn't just general clogging.