Your choice of high quality games is extremely limited when you have only 16K of memory available - as you are cartridge based. Which is fine if you are happy to purchase all the cartridges available - but there are 32K and 48K/64K games which will be your worth while to spend time on.
Back in the day - you would get the extra memory fitted first, and go for the disk drive option - to give you access to the disk only games and fast loading times.
Nowadays you can bypass the disk drive altogether and go for a flash cart instead. There is a guide to them in the forums somewhere. I've only got the Atarimax carts with it's programmer so you can pick and choose what games to put onto a cart in compiliations. These carts are rewritable many times over (using a laptop PC or whatever) - other more recent carts use SDcards - but may not support the bank switching as in the Atarimax cartridge system? It's easy to get a hold of someone else's compiliations to instantly build up a library or sample the most popularly played games.
I can imagine it's the easiest and less expensive way to play most of the games available and that buying individual carts is really only for the dedicated collectors out there.
It is a big wide world out there - of games, demos, utilities and what have you ...