One piece of advice I've always seen thrown around ever since I took an interest in collecting is:
Go for what you actually, really like. Don't collect things just because they're rare or valuable. It's one thing if they actually mean something to you, but collecting just for the sake of rarity will have you questioning your actions sooner or later.
I'm quite atypical for a collector, but take me for example:
I have absolutely zero interest in 75% of the G1 range. I do have a couple of rare ponies I'm interested in, like Stockings, Baby Princess Sparkle, Baby Sweet Tooth, Baby Honeycomb and especially the 7 Tales ponies (already have 2!), but that's pretty much it. Other "extremely rare and desirable" ponies, like Mimic, Nirvanas and the like just hold no appeal to me. Oh, I wouldn't mind taking them in, yeah, if they knocked at my door or something, but to me they're not worth pursuing and spending upwards of $60 on.
Come to think of it, my collection is currently at a humble 47 (well, humble in comparison to some other people's collections), and I already feel I'm almost at my goal. 30-40 more ponies, tops, and I'll be done. Which... doesn't bother me at all, actually. I do have other collections I want to continue... or even start.
Bottom line, what matters the most is how much
you like the item, and whether
you feel it's worth the money.