As you have a combi already fitted, you should have enough capacity for more radiators (as your boiler should modulate it's output dependant on the demand placed upon it).
Boiler output when combis are meantioned is completely irrelavant, as domestic hot water flow rates are quoted as priority (so a 30 kW boiler would be installed to gain a good DHW flow rate, even though you only require 15 kW to heat the house when in ch mode).
However, combi boilers are always rated at a set capacity for domestic hot water output (i.e. 14 litres of hot water at a 35 degree celcius temperature rise). If you are already having problems with flow rates, my guess is that a larger capacity boiler may not help you here. You are always restricted by the incoming mains pressure and flow rate (i.e. pointless having a 16 litre per minute combi if you are only getting 12 litres per minute from the mains).
The dynamic and static flow rates of the incoming mains water, needs to be tested and therefore it would be hard to even guess exact works cost without someone checking this.
TBH, £500 is not a realistic figure for the boiler move and more radiators. I would imagine a CORGI registered installer would charge a lot more (i know we would).
No-one can really say without seeing it first. Get someone to come and quote the job. Most people will come and do a free quotation.