LFS is probably one of the best simulators out there for disk usage. The core is only about 150 MB, and if you disable skin downloads, that won't grow much more than maybe 1 MB if you go and get A LOT of setups.
As for lagging computer, the only way that would happen is if it was running. A common misconception by a lot of people is that simply having a lot of applications installed causes slowness. In fact, the slowness only occurs due to lots of applications running. Potentially disk fragmentation can also be a cause for slowness, but that's a simple fix (although most won't notice any improvements by defragmenting).