you are all there as a bunch of mates is the assumption, the second anyone pays (for anything other than chipping in for a pizza or food at lunch) you will need insurance.
We play on private land fairly often with RIF's and have never had any issues.
Everyone SHOULD wear eye protection, masks aren't needed and you can't enforce the use of them, if anyone removes their own mask, it is their own fault if anything happens to their faces. Make sure you clearly state that before game commences.
Probably best to avoid the police at all costs on this matter, if they arrive with guns nd shiz, just call ceasefire and put all guns down, then go talk to them. If the site is large enough to play airsoft on, it is large enough to not be overlooked by the scaremongerers (funpolice) in your local community