MAC brushes are def. a very good investment. I always want to buy fun things like lippies & e/s instead of functional things, like brushes. But when I have resisted that temptation and forced myself to buy them, I NEVER regret it and find myself wondering how I lived so long w/o them! Anyhoo, to start out, I would recommend....
#252 (Large Shader Brush) = $29
#222 (Tapered Blending Brush) = $28
#187 (Stippling Brush) = $42
Subtotal = $99
(w/tax) Total = about $107
252 will give you a great all-over lid brush which you need. You also need a very good crease brush. I have bought the 224 & 217, but the 222 is way better IMO b/c it's stiffer and you have more control. Last but DEF not least, you need the 187 (that's just a given). I'm sure you've noticed how many people are IN LOVE with this brush, and it's for good reason. This will give you a brush that is amazing for blush, MSFs, bronzer, Studio Fix, etc. If I had NO MAC brushes and $100 to spend, I wish a MAC MA would have recommended these 3 brushes to me. HTH! Have fun!
Later on I would also add...
219 - awesome for smudging your eyeliner!
239 - smaller shader brush great for really packing on color
316 - a lip brush lets you get such a great, smooth l/s application
242 - fab brush for either dotting on concealer or applying paints
129 - another all-round good face brush, good for applying powder fndtn or really packing on blush.