your boiler is getting on a bit now but saying that along with vaillants worcesters are the best boilers on the market, theres plenty of gas engineers out there who can repair them and as long as the spare parts are still available it should give you a few more years of service while you save up your pennies for its eventual replacement.
the one thing you need to remember when or if you do replace it, if your system is sludged up then the new boiler will get sludged up too, so it will need a thorough flushing, thats the installers responsibility though.for a new diverter valve fitted your loking at around the 150 mark.
condensing boilers only save you on gas, not really going to save you on water useage.
cant guarantee that the new boilers will be trouble free as they are getting more and more complex and technical all the time, so more things to go wrong, but with any combi boiler you should get it serviced annually and expect to need a repair every couple of years, just like you would with a car.
your going to be looking around the 1500 to 2000 mark for a replacement boiler, dependant on what make and model you opt for and also if your valves or controls need replacing to comply with the energy efficiency regulations.
it may be worth contacting your council and see if there are any grants available in your area to upgrade to a more energy efficient boiler as there often is.