I'm not an expert, but if your wheels are a few years old and the rims are worn, isn't there a danger that hydraulic rim brakes might crush the rims? Besides, if you get hydraulic rim brakes aren't you just going to forever wish you'd gone the whole hog and got the disks?
Add it up. The difference in cost between hydraulic rim brakes and disk brakes, not including the wheels, probably isn't that much. The only big difference is the cost of new wheels. However, using rim brakes of any kind, let alone hydraulic ones, will wear out your rims and you'll need new wheels anyway! So you could end up spending the price of disk brakes anyway and still only having rim brakes.
So personally, I'd bite the bullet and go for disks. Either that or, if you think they'll cope with what you're going to do, just buy new pads for your 'V' brakes. Or buy a new bike! ;0)