Hi Bazzy,
If you ask 10 plumbers you will most likely get 10 different boiler models
However, I can safely say that you ant go wrong with Vaillant boilers.
An acreddited Vaillant Installer can supply 10 years of warranty that can be achieved via fitting the Vaillant proctection kit or you can buy it.
I would always go with the protection kit which is a brass magnetic filter to keep the central heating clean.
The aftercare is top notch never had any problem with Vaillant. I can highly recommend them.
The parts inside the green IQ 627 are brass and Stainless steel but not to forget at the end of its life the boiler can be recycled to 85%
The combi storage has the same brass and stainless steel parts.
Me personally I would go with baxi, biasi, ideal but that’s my personal opinion.
Vaillant
Viessman
These are the ones I would go with. Vaillant is market leader so I hope that will answer your question in regards to “ how do Vaillant stack up against ....”
Both viessman or Vaillant are great boilers. If you had the space I probably would choose the green IQ 627 with a cylinder.
The boilers I mentioned are bale to modulate which means these boiler wont use the full amount of kilo watts if they do not require. For instance in the summer you may need 4-5kw to heat the hot water that means you will Safe a lot on your gas bill.
These are very quiet running boilers. My personal heart is with Vaillant but if you ask some other engineers you may get different opinions.
If you want something good ( Vaillant or viessmann).
About the prices I cannot give you an answer as it depends on many factors.
Where you based ?