Welcome to Neonmob! I'm a traditional artist and I've created a few decks here, so feel free to contact me if you have any questions
Definitely scan at a higher DPI, I usually paint at sizes between 2.5"x3.5" to 8.5"x11", so I tend to scan at 600+ so I can print things a little bigger if I need to. Blowing things up will make any mistakes you might've made more visible though, so keep that in mind. It really depends on your technique/media.
Seconding what kararedwing said; Places like Staples, other copy shops, libraries etc can be good bets if your home scanner isn't cutting it. I go to a local copy shop for scans on the rare occasion that I create something larger than about 8.5"x11", which is about the max size my scanner can take.