IIRC partitioning just the first 4Gb will work fine, though it'll let you try more. If you use more there can be problems with writing data to the wrong place on the disk, but I don't know how often this causes real problems.
A 4Gb drive off ebay will cost even less than any new disk (I'd guess at under £10 inc P&P). With light previous use a good quality drive should last longer thanks to the slower spin speed (not an issue on a Amiga, really) ... unless you're getting one of the new fluid-bearing jobbies. In an A3000D this is better thanks to it running cooler!
Using a small amount of a much larger disk will considerably reduce seek-times as the head doesn't have to move so far (ie. get a cheap 160Gb drive, and even with OS 3.5 allowing the use of the whole lot it's best to only use what you need!).