Upgrading the Ti HD is easy: The main thing is not to stick too much money in an old laptop. So, the best HD means getting the mostest for the bestest price. You can go to dealmac.com and find HD deals, and you can go to the shopping sites and do your searches. Mainly, it's just finding an HD on sale, Seagate or Hitachi, 5400 rpm is the minimum speed (namely avoiding 4200 rpm like the plague.)
Buying an optical drive and sticking it into an older laptop is not the best idea, unless you've got money to burn. You're better off buying an external firewire 400 case and putting a 18X Pioneer burner into it. It'll be much, much faster than any internal Superdrive (think 6 minutes vs 25 minutes) and it'll do dual layer. Plus, you'll be able to use it with your next laptop. You should be able to get case and drive for under $100 (try newegg.com.)