An appliance place will often have skilled people who have experience installing new and retro-fit wall ovens.
The Big-Box stores will have some guys they hire cheap who may or may not have that level of experience--so, your mileage may vary.
Rough rule of thumb is that your oven cabinet will be 3" larger than the "class" of the wall oven used. So, a 27" cabinet will accept a 24" (wide) wall oven; a 30" cabinet will accept a 27" class oven unit.
This makes the shopping easier-- you do not have to worry about 30" wall ovens if the cabinet is only 28" wide.
Once you settle on a short list, go online and look up the Installation Instructions. These will then give you the next two critical dimensions--the height of the opening, and where the power needs to connect (most ovens fit in 24" deep casework, so that is less critical). It's a hugely good idea to not the minimum length of the MC/BX needed to make up the electrical connection as well as it's location--these can be a pain.
Ok, that's the architect-builder 2¢ for you.
For the foodie/chef side, allow me to aver that a double oven (or oven & warming drawer) combo is far better than nearly any m/w-oven combo. First off, the m/w is always where the oven ought to be, and it's a poor use of the volume it takes up, too.
I much prefer to have a purpose-picked location for the m/w--typically next to the coffee preparation area. I personally want them between 54" and 60" above the floor; but, I've no shorter users, either.
Which is why I like the "under-counter" installations. Gets the m/n off of the counter, available for many users, and the like. The m/w can be built-in or just on a shelf--which works out for being about the same size as a roll-out trashcan, too. Which is really handy for wrappers, popcorn bags, etc. Under that under-counter m/w is a great spot for a drawer to hold all those m/w-related accessories, too.
That's my t'pence twice, for what that is worth.