I think that it first depends on what type of products you use more frequently and what you think you’ll get the most use out of. Next, try to see what specific things would be unique to your collection (in terms of shade, finish, or formula).
MAC recently revamped their entire brush range and are now totally synthetic, though the prices are the same as before. A lot of people who have compared the new synthetic versions of old, natural hair brushes have said that they don’t perform as well, so that’s also something to consider. I own very expensive brushes and drugstore brushes. So far, I feel like if you want to invest in natural hair brushes, Japanese hand-made brands like Hakuhodo, Wayne Gross, or Chikuhodo are the way to go. If you only want synthetic brushes, I really don’t think it’s necessary to buy expensive ones. Elf and Real Techniques make great brushes, Sedona Lace is also good and they always run promotions, I’ve heard great things about Zoeva too.
Personally, I think you can skip MAC eyeshadows. Most of them are good but so many brands within the last few years have come out with better formulas that compete with MAC’s or are just better overall. My top picks are Uninterrupted, Kid, Soft Brown, and Typographic (it’s a soft black great for smoking out the lash line, not ideal if you want a deep black). Worthy mentions are Shroom and Deep Damson. Though Shroom is very dupeable and Deep Damson is a beautiful color but requires more work as it can go on a little patchy and is a little harder to blend. MAC’s eyeshadows are not always ultra pigmented and “buttery”, but I don’t believe that that means it’s low quality or hard to work with. But it’s worth mentioning if you prefer opaque, strong pigmentation like Lorac, Anastasia, Urban Decay, etc.
If you’re in the US, check out Nordstrom Rack, a lot of MAC products are on there right now!