If £700 is your total budget including the PC itself, forget it.
That mony could buy you a Sweetspot card to feed a scart RGB in. The same card will do component input but you have to change a dipswitch so major headache to keep doing it. Two cards would be needed. The card can also be set up to take s-video and composite.
As Nic says the only digital input you could get would be SDI, pointless unless you have SDI equipped kit.
All these could be run from within dscaler which would give excellent PQ. The PC graphics card will do deinterlacing.
Each Sweetspot card costs about £150+ so for £700 you could install 4 - one each for RGB, component, s-video and composite; or 3 and a SDI card if you have the need. Then a TV card and other bits and pieces - you'll need a MoBo with about 7 PCI slots
So if you don't have a PC to start with, you're looking at at least the money you'll pay for a good scaler. And you can spend limitless hours setting it all up, then again when it crashes, then again when you just know that tweaking seven of the filter settings in Dscaler will squeeze out a bit more quality.
I slightly disagree with Nic in that if you do buy good quality gear and spend the time setting it all up then PQ will rival the best scalers. But the hassle is really only for enthusiasts. The boxes are more reliable and easier to use if you're not a computer geek. And if you buy from the right place you'll get a nice man like Gordon or Liam to come and set it all up for you. Takes all the fun out of it of course but you will get the best possible PQ for your money.