Afaik there aren't any 26" mtb wheels that are significantly lighter than the typical low/mid range stock wheels at ~2100g/pair (without skewers, cassette, tyres or tubes) that come with many such mtbs. Exception for rim brakes that stands out in my mind is perhaps the Crossride UB at 1700g, but they are £140 or so a pair. The problem though is it has no disc mount so may not be portable onto your next mtb. The UB Disc version is 1900g, which negates the purpose somewhat. Most sub-1700g 26" wheels, custom or factory, start at ~£300 rrp, although you might be able to do better in sales.
In your case since it appears to have 7 speed at the back you might also want to check whether you have freewheel or cassette, freewheel wheels are usually heavier than cassette wheels (including the sprockets).
You didn't say why you want to lighten up or what you generally use the bike for. In any case the first step to lighten up the bike is to know what the existing components weigh, that will give you a baseline and let you know what cost delivers what weight saving.
For most people the best value is achieved by getting lighter tyres/tubes. That I guess is where answer to your notube question may become relevant. Using sealants instead of tubes does allow one to use thinner, light (most use non-UST, because UST tyres are heavier) tyres at low pressure (which is particularly beneficial for speed off-road) without too bothered by punctures. At the extreme this will allow you to use crazy light mtb racing tyres like Schwalbe's Furious Fred or some Maxxis models at around 300g each. Generally the sealant is not much lighter than light tubes such as Maxxis' Flyweight so weight is not the main motivation to go tubeless per se, and they are messier. You don't have to invest in tubeless rims/wheels to go tubeless - look up ghetto tubeless.
If instead you are using the bike mainly for road then a light tyre running at higher pressure might be the optimal solution. Schwalbe Kojak Folding 1.35 is also around 300g a piece, but as the name implies it has no tread whatsoever.