Find your most local site and go and do a hire for the day.
There is no point in buying a gun until you have the ukara. And there is no point buying the cheapest guns you can get because the internals are generally very poor. Once you have the Ukara ask again about guns, but instead offer a budget. So what can I get for <£150 that has decent internals ect.
Site rule change from site to site.
The ones that seem to be universal are:
You must have boots to play. They must offer good ankle support. They don't have to be army boots.
You can never under any circumstance remove your eye protection. If it comes off you must do a man down call and have a marshal help you.
You must always take your hits. Airsoft doesn't leave a paint mark. You get unlimited respawns in most games. If you get hit you take it.
If you own and use your own gun, it must fall under the sites own FPS limit. All guns will be checked. This limit moves from site to site.
Loadout wise, Anything goes. Some like police style kits (all black), Others like military kit. Jeans and a dark hoodie looks a little odd at some sites but if you are hiring they normally provide camo overalls anyway. Current MTP Brit loadout is very common and you can get army surplus at reasonable prices and the kit is easily durable enough for airsoft. Good boots are more important than anything else, consider them as safety equipment. The current issue Alt berg defender can be got in army surplus for about £35 in the worst condition, £50 in reasonable condition. Get a pair, wear them all the time until they feel like slippers. You can also get an osprey style plate carrier to keep your abdomen from taking shots if it's sensitive.
A typical Hire will contain. Gun, Spare magazine, 1000+ bb's, Overalls, Eye and face protection. It's everything you need for the day. Extra ammo at most sites runs at about £10 for 3000 bb's. Some but not all sites provide free tea and coffee and some provide a lunch time meal.
Lastly your medical condition. You can play at your own pace. Tell the marshals that you have an issue and explain it to them. Personally I'm a lazy player that never runs, It doesn't slow down my games. Airsoft engagements happen at decent ranges, you will always be looking for cover and using it. If you can't run it'll make falling back harder, but it isn't going to stop you been able to play effectively, and you don't need to go the sniper route. Airsoft sniping also isn't what you expect, It involves more moving and running than normal play to get into position and fall back, And is physically demanding with all the bolt cycling. You will see this on your first hire day.