Wheel advice (11 speed) for lightweight rider

I can recommend DCR wheels and The Cycle Clinic for handbuilts.

Ryde Pulse Sprint rims may be a good place to start; they are about the lightest aluminium rims around at the moment.

DCR will do you a set of those with his own hubs (which are excellent) and CX Ray spokes for well inside your budget. His quoted weight for 24/28 spokes is 1330g. You'll be fine with a lower spoke count so you'll be able to go even lighter than that.

Not everybody's cup of tea, but Mavic are worth looking at for a decent factory option.

I've run a set of Elite S for a while now and they are a great all round wheel.  Just bought a set of Ksyrium Pro SL's for under £600 and first impressions are good.

I'd definitely look at Fast Forward F2As. 

Lovely DT Swiss 240 hubs, wide but light rims and good reputation for build. 

Weightwise much the same as the others mentioned but great quality

Would just like to add that while I'm normally on the "saving a few grams here and there is wildly overrated" camp, for someone doing the Marmotte at 7 stone I'd make an exception and recommend trying to cut bike weight. Although, on the other hand, high-end alloy rims aren't really all that heavier than mid-to-low end carbon rims, particularly low profile ones (unless wicked, windy, winding descents on wind-catching deeper wheels is something you're interested in), and as you know they usually do offer better, cheaper, and quieter braking.

Custom built with Pacenti rims, Tune hubs, Sapim CX-ray spokes should be do-able on your budget so 1300g ish wheels, tubeless compatible if you want to try it but wide and comfortable if you don't.  Chris King hubs are both heavier and more expensive than the Tune I think so probably out of your budget.

Benefits over RS81s etc. would be weight, comfort, longevity and maintainability.  

On the downside you won't have a reason/excuse to buy new wheels for a very long time indeed!

Or ignoring the clincher part of the brief, as they're summer wheels, get carbon tubulars and go even lighter and more aero. DCR referred above have a set within your budget (just).  Or Far Sports, cheaper but not everyone is comfortable with buying from China.

Lightweight wheels are nice but bear in mind that taking 200 grammes off your wheels won't make any difference to the time it takes to ride up a hill.

I'd not be obsessed about it, get some handbuilt wheels from Strada, Wheelsmith or one of the builders mentioned above.

Being a climber your looking at as low wright wheel as possible, I'd say go down the hand built route as an experienced wheel builder will shed 100s of grams on comparative factory wheels.

+1 for handbuilts. You could think of super lights at your weight, if you want them. See e.g. http://www.spadabike.com/it-it/15/stiletto/ 1260g for a few quid under £500.

Get some hand built, look at just riding along website.
http://www.justridingalong.com/

I got some lovely mtb wheels with hope 4 hubs, sapin spokes

if you buy the dura ace or handbuilt, then in future you need only replace rims as they wear out, but if you go to the zondas or similar the wheels are not so easily repairable.

for the 700 budget you are getting pretty close to getting a pair of chris king hubs with HED or pacenti rims.

what do you mean by min weight? I;m aware some wheels have a maximum weight rating, but I've never heard of minimum weight.

Hand built would be my advice - talk to David at DCR Wheels. I got some lovely light and stiff (c 1400g) alloy clincher wheels from him, easily within your budget. 

What do you want from your new wheels?

Lightest possible weight? Best aerodynamics? Easy to service at home? Readily available/economical spare parts? Nice ride? Good looks?

I normally try and identify something I want to change about my bike or riding experience, then buy the bit of kit that best helps me realise that change.

Unless it just looks really cool, which is just as good a reason to buy something as any other