First off, great purchase. I love my DM7s as much as I love any other marker I own.
A couple things I recommend:
1. Read the manual.
2. Understand the manual.
3. Buy a medium parts kit. Unfortunately it's hard to find the DM7/PM8 parts kits now so you can just buy the DM8-11 medium parts kit and then just understand the eye pipes won't work. So pick up an extra eye pipe for a DM7/PM8.
4. Spend 5 minutes after each weekend of play and clean, inspect and lube the bolt o-rings.
5. Once a year clean and rebuild the LPR and HPR.
If you have any issues with your DM7, first ensure you're using a good quality battery, I use Duracells and they work great. Second, if it's leaking, look in your manual or read the sticky threads here, but this shouldn't be an issue if you've lubed it properly.
Since you bought used you might as well go ahead and clean, inspect, replace if necessary, and lube the bolt o-rings to get yourself ready for your first day of play.