you could replace your back boiler with a condensing non-combi boiler but if your cylinder is an old type you wll have to replace that as well with the latest high heat recovery coil foam lagged type, thats gonna add about �500 to the cost. you will aslo have to have thermostatic radiator valves added to all but one of the radiators and will have to have a cylinder stat, room stat and a programmer if you dont already have them to ensure full boiler interlocking, this is all required by law to comply with the energy efficiency regulations. this is the reason installers prefer combi boilers, they are cheaper and easier to install. of course if you already have the above then you only need a new boiler.
if you opt for the combi boiler route then you need to be aware that the boiler power output you choose is dependant on the hot water you require. Combi boilers never give as good a hot water delivery as you get from your baxi so you need to go with a high powered combi or you may be dissapointed. i would suggest a minimum of 28kw and preferably 30 or over. but the higher the power the more it will cost to run and you will also need a larger diameter gas supply pipe which will add significantly to the cost of the installation.
if you decide on a new boiler or system then get some engineers in to give you some quotes and discuss the different options.
back to your baxi back boiler, don't worry about running it too much, gas is so expensive at the moment it's going to cost a fortune no matter what boiler you have.
and if you do want a new fire for the front your very limited in choice, you will need to contact baxi to find out whats available but they will need to know the model number of the boiler when you ring.