Alot of time you can call a local mom and pop computer shop and get a 10gig hd from them for really cheap. Also try calling a local university and ask the CS department if they have some for donation.

Also on a older machine you would want your swap as the first partition allowing for faster access increasing the speed of it.

Dual booting with a fat32 partition allows for rw access from linux as well.

Another though is that if you alot the space (resizing it) and leave in un-partitioned I belive the newest Debian will detect windows, use the remaining space without touching the fat32 partition and even enter a entry into fstab so that it is auto-mounted each time.