All sites are pretty much on a par for pricing, ignore the fact that some advertising says that you get overalls, goggles, gun and air hire for 'free' because you get it for 'free' as part of the package anyway - you can't play without them.
The worst and most expensive is the starting price with 100 paintballs as you then need to buy them as you go along
The best way is to prebook a package, e.g. Half day with everything plus 600 paintballs for about £35 per head
http://www.bricketwoodpaintball.co.uk/paintballing-packages-prices.html
Generally ignore 'upgrades', if it's a hot sweaty day then a better lens will help, but you can deal with a normal lens if you use a bit of antifog and don't block your airflow with big neck scarves and hoods. A 'machine gun' upgrade gives you something that looks more like a gun, but is pretty much the same as the standard gun. The difference is the player thinks they have something better, and that less people have used and abused it in the past. (The extra cost of the 'better' gun is pretty much due to it taking more maintenance effort and staff time than 'standard' rental guns)
I can recommend bricket wood as linked above. Look at their website then give them a ring and chat about what you are after from the day
See also the 'what paintball costs' thread for general information