Suffice to say that this is a major can of worms both technically and logistically but if it's done right it's also terrific fun.
The techniques involved in micing an orchestra and choir outdoors for PA and recording are generally very different from any normal indoor recording job. Unless you have a very special (read unfeasibly expensive) stage built, normal ambient orchestral/choral micing is not going to cut it. It can be helpful as an addition to the balance but to get the kind of control you need for PA and to help get reasonably clean sound for recording you'll need to adopt a more "studio" micing approach - i.e more mics and closer.
Technically, you have to overcome the complete lack of any usable acoustic reinforcement/support of the sound (apart from some horrible upper mid/HF slapback and tizziness from the normally more or less LF transparent construction) and the spill from the PA system causing feedback/colouration.
Recording and micing/mixing for PA on an event like this aren't mutually exclusive but you wouldn't want to try and simultaneously do both yourself. It's hard enough getting a decent PA balance without worrying about the recording. You'll almost certainly find that you need a different balance for each and though they will be fine sharing the main micing, each will have slightly different requirements when it comes to spot mics.
Logistically, someone - presumably not you - will have all the licensing, fire, police and ambulance service cover and access, venue approval, security, parking, catering, toilets, physical construction, and a host of other things to think about.
From your point of view, make sure you have plenty of help. It's important that there are enough people available and that they know exactly what their role is and how to carry it out. And put in place a simple, clear structure for who is responsible for what and make sure everyone is aware of it.
Some walkie talkies will be invaluable for communication on site, as will a wired comms system between mix position and stage (walkie talkies become very hard to use when the PA is up loud). How complex the comms are and who needs to be on it will depend upon the size of the site and the layout of the tech side of things. e.g., if the amp racks are under the stage it's a good idea to put a comms point next to them for emergencies.
Don't forget to allow for Sod's law coming into play and the resultant high winds and/or torrential downpours trashing everything. It sounds simple but providing weather protection for mics that lets the sound through whilst minimising the effects of rain and wind is not easy (unless you want to spend real money) and is definitely not cheap! ordinary foam pop shields don't do anything like enough! You also have to protect the other electronics like amp racks, splitters, mix position, and cabling against wind blown rain or they won't last long! Make sure that all this and how/where the speakers will be mounted is taken account of in the stage design and planning. Also, whoever builds the stage will need to ensure that all safety regs are complied with and pay proper attention to things like safety earthing on metal structures. None of which should be undertaken by well meaning but inexperienced builders/scaffolding companies. Use someone with experience in this kind of work. At the very least it's embarassing when an amp rack gets wet and packs up but when it takes a chunk of stage up to mains potential, at worst its lethal.
On the subject of power - make sure you have enough! You aren't going to run a show like this off a single 13A socket extension - make sure you do some proper load calculations and then allow a healthy margin for over-run. And just to stress again - an outdoor stage is a very hostile environment for audio/lighting equipment. Make very sure that everything is done that can be done to ensure the safety of the crew, performers and audience. Don't take any shortcuts and if in doubt about any gear that a band might bring along don't use it. It only takes one dodgy lead or amp to cause real problems. Make sure that everyone is aware of the safety implication of what they're doing.
And get plenty of insurance!
One other thing is that it's important to make sure that everyone's expectations are known and understood. As the general technical standards of live shows continue to rise, the expectations of both performers and audiences are rising too. People are only too happy to let you know if they're less than happy with any aspect of the presentation from the lighting and sound to the catering and toilets available on site! As long as people don't come expecting the full 3 Tenors show with multi million pound production budget and technical standards everyone will be fine. There's nothing more demoralising than working your bum off against a limited budget, pulling off a miracle to put on a good show then having people whinge at you that it wasn't very good because you didn't have 50 varilights and a massive PA system.
Best of luck.
since this year it'll be up for 2 weekends.
But I will be the only engineer for this show to my knowledge. I will be able to get some people for tech to work as hands.. but I'm setting it all up my self. 