A vertical flue will notch the price up unfortunately. Have you space outside for an External Oil Boiler? As that will probably (depending on pipe alterations) be cheaper than an internal with a vertical flue, extensions etc
Worcester or Grant are the best oil boilers imo. Worcester have just won Which? Best oil boiler for 2016. 7yr manufacturers guarantee on Worcester oil when installed by an accredited installer. We have a Worcester danesmoor system 25/32 which I expect from what you've said is the size you would need. They do a regular version as well, which if your pipe work is old would be the best option.With System boilers the heating system is pressurised so oldish joints may not hold up, which would cost more money, which you don't want, although you would loose the F&E tank in the loft and the possibility of loosing heat up the open vent.
Always get 3 quotes, anymore and you will confuse your self.
By getting an accredited installer you also have the assurity that they're OFTEC registered (so workmanship warranted for 6yrs against non-compliance, even if the company no longer trading) and installation registered with Building Control, they have trained on that particular manufacturers boilers and can give the maximum guarantee on the boiler.
Oh and finally, I know Worcester have their own 'in guarantee' engineers who carry all the parts, but I'm afraid I don't know the current position on Grant. They used to rely on local engineers attending, diagnosing and then ordering the part. Not sure though so please check. My mum has a Grant external, great boiler, about 5yrs old now and never gone wrong.