I have been looking around, and could get a set of 50mm carbon clinchers (gigantex) built around a G3 for c.£1500
Looks to me as though you could get an SRM power meter built into Rotor 3D+ cranks for around £1900 or Rotor's own power meter with your choice of round or Q-rings for around £1630, just doing a monetary conversion from a direct order of either off Rotor's site. Imho, both those choices would be far superior to a G3 laced onto Gigantex rims. IME as mentioned up thread by Tinea Pedis others you are going to want to be able to swap out to non aero wheels in some circumstances. Gigantex rims aren't the best either.
I'd invest the significant portion of my budget into a crank based power system if you are at all serious about racing. I don't know anything about Power2Max. Early reviews had some issues that I understand have now been addressed, and it's an even cheaper option than the two mentioned (SRM, Rotor). You could probably also get a used Quarq. All of these are the best choice imho as compared to trying to do an all in one wheel set.
However, it is a fine choice to simply lace a G3 into an AL rim and be done with it, at least having access to power-based training, on a wheel set you know you can use in all conditions and easily service. Your options here would be to eventually move over to crank-based power and get a couple wheel sets, get a used set of aero wheels to race on in the meantime (you wouldn't have power data from races), or get a cheap set of carbon rims (check out the Far Sports thread).
Any or all of this in one or several steps I feel is better than trying to do all-in-one using a Gigantex rim. If you were going for the two birds one stone thing then I'd do it with ENVE rims, but even then I would choose an above option vs building a PT into ENVE rims. At some point if you are serious about power and racing you will want it crank based. I'd simply start there - or - do the cheapest and most functional PT build on AL rims as a temporary holdover until you can go crank based. Imho.