If you have unlimited wealth, I'd go with the Lost Desert collection, comes with an amazing avatar. Chances are you'll have a lot of trouble finding some of the more expensive avatars.
If you're looking for an easy challenge, go collect the shells, I believe those are quite cheap.
If you're looking for an avatar set, your best pick would be the Battledome set, it's the cheapest, around 170m. Other avatar sets have very high end retired or limited stamps. If you're just looking to increase your numbers, check out the list here: (you need an account to see links)