My first post on here, so please don't bee too brutal

I have an old kitchen that i'm looking to renovate and do most of this myself. I'm going to keep the unit carcasses and replace the doors, drawers and panels where possible. After measuring the old doors i've noticed that some door unit differ in height from the standard pre-fabricated versions by as much as 15mm in some cases, though often it's around 5mm too long or short.
My questions are:
a) if the new door/panels still fit, is it better to get ones that overhang slightly than panels too short? My theory is the overhang will look less unsightly than the shorter ones that expose the carcass underbelly
b) should i ignore option a) and ensure all doors are an exact fit by ordering made to measure?
c) If the doors are slightly different sizes to the old ones, do i need to measure 'top downwards' for determining where the hinge holes should be? (ie - measure from top of panel to 1st hole centre, then 1st hole centre to 2nd hole centre)
d) are there any other gotchya's or tips i should learn to help when buying/fitting the doors?
I've a reasonable amount of all-round DIY skills (fitted my bathroom), but never undertaken something that has this expensive / precise finish in terms of looking right as a whole
Apologies if i missed a thread on this, but i could't find one. Please feel free to link to it, or any websites that have these tips and hints on
Many thanks for your help
