The most important thing of all to promote performance and reliability from a new combi is to ensure the system is as clean as a whistle before you put the combi in, and that you have the correct amount of inhibitor (Fernox or Sentinel) to suppress corrosion. We now send water samples to Sentinel for analysis to make sure we have flushed the systems properly, and yes..it took us a bit to get it right when we started sampling...at first we got rejects with "too much molybdenum" in the comments..don't ask me!!!
Also, if you are in a hard water area you need either a scale reducer..we us Scalemaster gold..the cheaper ones with pushfits usually leak after a year or two, or if you can run to it, a full water softener..which is what i have..but its pricey to buy and to run.
Then its just a matter of choosing a reputable make, of which there are quite a few. Alpha:Worcester Bosch:Keston:Buderus:Vokera are all names that seem to attract more favourable than bad comment. Sadly perhaps, Ideal seems to have knackered its reputation somewhat recently.
Every installer has their own favourites, so you will see heated arguments on this forum about "which is best".....and everybody is usually right
Alfredo