Current mtb cassette hubs generally can take a 7 speed cassette (usually requiring a 4.5mm spacer), so 4) is a non-issue.
But I think your remaining requirements are a little challenging.
The answer is as usual boiling down to factory or handbuilt wheels. For factory wheels Mavic Crossride UB, which cost about £150 a pair, would imo meet all your requirements (if £150 is considered adequately inexpensive). The disadvantage of that, particularly for rim brake wheels, and applicable to most if not all factory wheels, is if the rim is worn or damaged it might be more economic to replace the wheel than the rim.
For handbuilt the challenge is associated with hub selection. I don't know why having sealed (I presume that means cartridge) bearings is a key requirement, but the effect of that is it eliminates literally all Shimano mtb hubs, which are generally relatively inexpensive while of good quality.
While quality mtb hubs with cartridge bearings are not uncommon, many cost as much if not a few times more than the above wheels. The choice of low cost yet proven mtb cartridge bearing hubs is, afaik, very limited. Apart from using hubs from e.g. Novatec (a high/mid end offering by Joytech, a huge Taiwanese hub manufacturer, whose hubs are also sold as Spesh, Ambrosio, Superstar and others), which cost £50 a pair and up, it is hard to achieve a build costing less after including some decent double butted spokes, 26" Mavic rims and labour; and the durability of a pair of handbuilt is highly dependent on the builder's skill. You will be using disc hubs, but to me that is an advantage in terms of future proofing. Unfortunately Novatec hubs generally have alloy freehubs, so you also need to pick cassette with a bit of care.
Hope it helps.
