If you are a tinkerer, as i am, fiddling with guns to get more accuracy and range is all part of the hobby. It is true that cheap guns often have very inconsistent air seal through the whole chain, from Piston Head to Inner Barrel, however it is not the case that all expensive guns have better internals, and even those that do do not always produce as much consistency as you would hope for, for the money. On the other hand, if you set up the internals well, even the cheapest original gun can proper pwn one which cost 3-5 times more.
I would suggest that your best bet then is to buy a cheap gun, disassemble it, and learn how the parts work together. Then you will see the flaws in design and/or the compromises which have been made in order to accommodate other features and start to see how you could improve on what you see in front of you. You will probably discover that someone has thought exactly that before and produced an aftermarket upgrade which deals with that problem, or that somewhere online is a tech forum in which people have/are developed/ing a bodge fix, but hey, you never know, you may come up with a better idea...
Accuracy itself is a function of two things: consistency of air seal makes the power of each shot predictable, which is important for elevation; stability of back spin is necessary so that the Magnus effect is generated vertically - if the hop up is applied to one side of the BB more than the other, the BB's trajectory will curve to that side - aha, adjust for windage you say...? Well yeah, but for accuracy that curve must be predictable, and unfortunately there are a number of ways for inconsistency to creep in, either during the contact between BB and rubber, or as the BB transits the barrel...
Some of the best ways to improve accuracy are also the cheapest and, since hardly any guns come with them as stock, should always be your first modifications:-
CT-2 Teflon Grease - slap it liberally all over the Piston Head O-Ring and make sure it evenly coats the interior of the Cylinder;
Air Seal Nozzle w/ O-Ring - smear CT-2 around the rear 1/2 of the Cylinder Head spout so that the Nozzle O-Ring gets greased but the grease does not get blasted into the hop up chamber;
PTFE Tape exterior of the Hop Up Bucking (Hop Rubber) - a few turns which then improve the seal between the Inner Barrel and Rubber - grease the outside of the Rubber/PTFE Tape so that it slides into the Hop Chamber without sticking (which can tear the Rubber)
H-Nub (also called Fishbone Spacer or Improved Cushion) - this forces the BB into the centre of the Inner Barrel and improves the left-right balance of how much hop up is applied, but is not compatible with all Hop Up Chamber designs (G36 for eg)
W-Hold Bucking - in the absence of an H-Nub a rubber with a split bump for contacting the BB goes some way towards L<>R balance of applied hop and BB centring (there is also the Falcon Dual Point Rubber, but IME it does not lift BB's heavier than 0.2g that well / CYMA stock Rubbers have this type of split as standard, but they also suffer from poor quality control, so some of them are not moulded all that well - IME a good 1 is actually better than a PDI W-Hold Bucking) - these are the way forward in A&K SVD's, G36's, and any hop unit in which there is no standard Nub.
People, as much as we may be trying to help, to be inclusive, I'm not sure that we are being as sensitive to our new mate's situation as we would no doubt wish to be - I mean yeah, mounting a heavier gun on to his chair certainly is a possibility and other people have turned chairs into armoured vehicles, but I worry that some of the suggestions above are not only very expensive builds, but also have the potential to be seen as making tclancey a figure of fun. There is no reason why he could not take part in ordinary skirmishes, with an AEG like anyone else, so long as the site was chosen for being accessible to his all terrain chair, and with the cooperation of the organisers to bring some common sense to any problems which they foresee.
Clearly, tclancey, you are never going to be front line assault material, because you will not be in a position to dive for cover when the opposition spot you, but then again, many of us, through age, lack of cardiovascular fitness, and/or infirmity, cannot keep up with the yoot. Nevertheless we still manage to have a good day in supporting fire roles, suppressing the opposition so that nippier kids can get forward and get kills over the top/round the sides of cover, being still there when a rush attack has failed allowing our side to regroup and have another crack, and of course, where we tinkering old/fat/knackered players excel, defence (when our uber guns can pick the opposition off at ranges at which their suppressing fire is ineffective).
Edit to add: guns - for an M4 get a G&G, either a bog standard Combat Machine, or one of their Intermediate range / for a G36 the JG has parts compatibility with TM which is a bonus worth having if upgrading yourself is your plan, because although the SRC is a better gun, it isn't parts compatible with any other brand - the Umarex is apparently quite good also and has electric blowback.