- Will he accept used equipment

- Does he want it solar powered

- Will it need to run on batteries or is it going to run off the vehicle's 12v outlet?

- Is it ever likely to need to turn into a mini stage ie at night or at the end of a festival when the stages close? (obviously pushing it for 500 quid but could be done with used gear)

The simplest approach would be a car amp with built in adjustable crossovers and high-pass. If you get tops with a passive crossover then you can power a 1 sub 2 satellites system from an amp that'll be less than a hundred quid new. A real budget high powered option with all the features is the Bass Face amps, but they have reliability issues so I wouldn't recommend them for anything professional.

Vibe do some good car amps. I've used a Litebox Stereo 4 outdoors for several months at a stretch (literally) without adequate protection from the elements, and it survived. I'd probably look at their range and find the one with features most suited to the speaker setup you settle on. Not sure if they do a bass and two tops model.

Wiring it all up to the car (if that's the case) should be intuitive enough from the manual. If running from an external battery then you need to add a separate short length of wire to the amp between positive and ground in addition to the wires going into the battery. Deep cycle lead gel/leisure battery would be ideal, but these are fairly expensive at the size that'll last through a festival.

Helps to have a few spare fuses handy for the amp because they do blow now and then for reasons which aren't always obvious. If he's being kitted out from nothing and doesn't usually work with sound then a few spare everything (cables etc) would probably be appreciated.

Speakers, obviously efficient is good, but if it's just for background music then quality is more important and it might be preferable to drive more of the budget into the sub as it'll be outdoors, to make sure it actually has some presence. Some ceiling or wall bracket mounts so the midtops can be mounted to the right bit of the horsebox would be a good idea. You could run some basic lights (LED strip or something more fancy) from the 12v line. A bog standard Chinese inverter on eBay is about 30 quid and would be a must imo even if it's not part of the typical setup.

A 12v or 9v mixer like the Behringer Xenyx 1002 (about 60 quid new) will let him run a musician or small band through it with mic, line inputs with separate 3 band EQ on each channel, backing track etc, I'd definitely consider suggesting something like that. Being able to do announcements over a proper mic with an EQ band on the mixer to cut the bass has loads of possible uses for a pop-up thingimajig.

Helps to have covers or a sealable box of some kind for all the electronics, or some other way of waterproofing as it'll be outdoors so much and even with adequate rain cover, several days non-stop in a field several times a year will take it's toll.