I can sympathise with your dilemma, but are you trying to say that you will buy the boiler, have it fitted, then later buy some rads, have them fitted, or actually as Tony has interpreted it, buy each component, stock plie them and have it all done together later?
If it is the latter I would suggest saving until such a time as you can afford it all done at once. It will be cheaper that way.
Going back to your question, I would, as a mainstream boiler, go for the Vaillant Ecotec Plus range, probably the 831 or 837 if you prefer baths mostly and have at least 15 l/min flow rate. I also fit the Glow Worm Ultracom range as a budget boiler, and luckily, unlike some on here have had very little problem with them.
Existing rads can be used, but make certain the whole existing system is properly flushed and cleansed, preferably with a Magnaclean or similar fitted as well.
You may be unlucky enough to have a pin hole appear in an old rad, which obviously would have to be replaced as an extra afterwards.
The airing cupboard is quite often the best place for a new combi, but flue termination and access to the gas meter should be taken into consideration, as both these could add substantially to the installation cost.
As far as I'm aware and certainly with the boilers I fit, the warranty starts on the date of installation, within reason, direct from the manufacturer. The warranty from the supplier will be restricted to the date of purchase though.