Not had time to read all the posts, but have you considered considered asking your installer what he recommends?
Thats what I'd advise
That is what I said as well
I would be talking to friends and relatives for recommendations. Invite the person to suggest boiler and costing. He may not want to fit the boiler you supply
While one boiler does the same thing as the next, it could have weaknesses or failures only a service/ breakdown guy is likely to be aware of, hence he is the best person to seek out
Then you have the spares prices. Once the warranty is over (can be null and void if the installer does not go through the hoops and loops), some of the parts can easily cost a chunk of what the new boiler cost, so go cheap then pay through the nose for the parts. Do the check yourself- pick a cheap boiler and then look for what the fan, PCB or the heat exchnager costs
There are guys and gals out there who want to earn an honest crust, price is not adjusted by day of the week, amount of work in hand or car you have in the drive
Knowledge under belt will obviously effect the price as well professionalism
Replacement boiler is not a case of shoehorning the new boiler in place of existing with no regard to what is existing there at present
An open vented system, converted to sealed system could have sting in the tail later in that system now pressurised could weep from system weaknesses
System debris will almost certainly cause an issue to most modern combination boilers
A gas line feeding the present boiler may well be correctly sized. For new boiler it most likely would need upgrading- bigger the power, more gas required by the boiler
System controls can make an existing system more 'economical' to allow planned upgrade
Finally the installer. He will destroy a good appliance through bad workmanship or breath life into it regardless of the appliance selected
While not intending to blow own horn, recovered a boiler from that had been attempted at repairing (had some new parts fitted by that person then eventually boiler dumped) boiler was serviced and £5.00 diaphragm is all that was needed. Boiler gave 12 years of good service when it had to be removed as it needed a major part replacement