Yaaaarp. Bulldog are pushed hard by the BBgnuz4U vendors, I expect they've got a decent margin on them. That said, that one is cheap enough, has got all the required parts, and you can get a peach or a lemon from any brand. Most of us have spent more on worse, or will do. It looks like that comes with one midcap, so you'll either need a bunch more midcaps, or a couple of hi-caps to keep him in the game. M4 magazines are generally cross-compatible-
ish, but with foibles that sometimes require you to shim them out with tape or the fuzzy side of a velcro strip on the front or back to get them lined up and feeding reliably.
Are you OK for batteries? If you got nimh and a £20 "smart" charger pushed on you, well, that happens too. They'll work, and you can switch to lipo/li-ion and something like a SkyRC S65 charger later, if you stick with the hobby. I prefer to use pairs of lipos, in the 1200-1400mAh range, and swap them over at lunch.
My only real concern for a first day would be if you get one that's shooting hot, and get knocked back at the chrono. Probably not, but BBgnuz vendors shift so many that they don't tend to actually check them before selling.
Still, fingers crossed. Just clean the barrel thoroughly, and I personally wouldn't use anything less than 0.25g in woodland. I find that 0.28g is the sweet spot for price and performance,
if the hop unit can lift it. You'll find folk on site happy to let you run some of their BBs through it to find out, without committing to buying a bottle of your own first.
That two-tone broomhandle is utter heresy, mind, and might very well be shooting hot on typical "green" gas. I'm seeing it listed at "370fps" or 1.27J which would either get it knocked back or you'd be asked to use it with DMR restrictions, i.e. a slow rate of fire and a minimum engagement distance, which rather obviates the point of a pistol. Again, you won't know until you get it chronoed. If it is hot, you can try running lower pressure "144a" gas in it.
This is all sounding a bit doom-and-gloom, but in the grand airsoft scheme of things there's always a way to mend and make do, or muddle through, and once you get out there, it's all grins. If you run into any issues on site, just ask the other players or marshals for help - you'll always get someone on site (hello!) keen to throw stuff at you to test out or help out, up to and including whole guns. I tend to bring at
least two backup AEGs, shotguns, pistols and what-not.