DEFINITELY buy new forks, spending £200 on a new CSU for an old fork is poor economy. Especially when like you say, you can get a full new fork for £300.
What hubs are you using? Hope Pro2 per chance? If so, Don’t bother going down the QR15 route (unless you can get a fork really cheaply), get the 9mm QR adapters and a 9mm QR axle from Superstar (£7.99, a LOT cheaper than the DT one), this route makes a massive difference over a normal QR and for not much money at all and works with normal forks.
As far as going from 90 to 100mm of travel is concerned, I’d be very surprised if you really notice it. It’s not going to kill the geometry, it’ll slacken the bike by less than half a degree, if you feel the front end is a bit high after doing this, drop the stem down by 10mm of spacers to compensate…