I think Fishtaco is 100% spot on with his advice, but I could add a few other suggestions
1. Many people will warn away from high premium numis, especially coloured coins. Which is probably sound. But if there is a series you really like, what the hell, splurge. You only live once
2. I keep a spreadsheet of all coin/bullion. Otherwise sometime I forget what I've got, and I accidentally double up
3. YEnnus was also correct- it depends on your objectives. For me it's an investment, a store of value, but also some enjoyment along the way collecting. I am partial to kooks,ASEs, fiddies, PM lunars,10oz bars and kooks,and some predecimal silver.
4. There are too many choices out there, so unless you are a millionaire, you need to be selective. Also need to consider SECURITY and space. Many of those Numis issues have bulky packaging
That just my view.