I "overfill" all the time. If you have a check valve in your iar line you will not have beer backing up. Also, I do not purge after. I sanitize the keg by putting in some starsan, hooking it up, and running it through the beer line with COs2. that - "pre-purges" my keg before I put any beer in it. Then I fill it with as much beer as possible and close it up.
On Tap:Foreign Extra Stout, ESB
Fermenting: Fools Gold IIPA
Up Next: Oatmeal Stout