Whether selecting alpine, touring or snowboard boots I really cannot suggest strongly enough that you go to a boot fitters with a wide range of boots. Most resorts have somewhere good, Footworks or Snell in Chamonix for example. First a couple of tips, a boot that seems comfortable and fits well in the shop is almost certainly slightly too large. The less experienced a skier, the larger the boot tends to be! Secondly buy the boot that suits your skill range and fits your foot the best. Do not think about model, mark or gadgets. Start with the best fitting boot you can find, ski it and only then think about making further adjustments. Although a boot fitter can do a lot with heating and padding there are certain adjustments that cannot be made to a shell. In general a shell with fewest rivets and holes is better. I have downhill and ski touring boots and have made no adjustments to either, no canting, alignment or custom foot beds, and I don't find it that easy to get comfortable city shoes.
Remember that the inner boot will deform quite quickly and give a sloppy fit, now I could ski down a black piste with a boot two sizes too big and with all the buckles undone, but off-piste you need a boot that holds your foot. Semi-soft boots have appeared on the market, these promised to be a revolution for comfort but the lack of control means that designs have not really taken off.
The first thing to do is see if the shell is the right size (the famous "shell test"), take the inner boot out and put your bare foot in with toes touching the end. You should just be able to squeeze two fingers behind your heel (about the width of the inner boot plus a single ski sock). With your foot centred in the boot three important measurements come into play, the width, the height and the space around the heel. At no time should any part of your foot touch the shell but the volume should not be too large, the heel should be held firm, if it moves you will have to over tighten the buckles which will be uncomfortable on the shin.
Remember that plastics change with temperature. A boot that is flexible in the shop with be stiffer in the cold of the ski pistes. This is why I suggest buying a boot at the start of your stay in resort and then getting it fitted over the week (assuming you are on vacation). This is also a good excuse to ski a lot. Visit every shop in the resort over the first couple of days to hunt down your ski boot if needs be.
Injected inner boots should be left to experts, they can often cause more problems than they cure, however lightweight thermoformable inner boots are worth considering. Most alpine ski and ski touring boots have a thermoformable option or they can be purchased as an aftermarket accessory. These are more comfortable, warmer and much lighter, an important point if ski touring. The main difficulty is with sizing thermoformable liners, the inner will adjust but it cannot pack out a boot that is too large. The boot will not be truly comfortable until the inner liner has been baked. Experience at PisteHors.com says that a thermoliner that is uncomfortable in the shop (toes cramped) won't pack out enough to be comfortable when baked. A shell test is more important than ever.
For obvious reasons hire boots cannot be supplied with thermolinings which is all the more reason to get your own boots. Don't leave your thermoformable inners near radiators or stoves to dry or they risk deforming.
I wouldn't bother with a custom foot bed unless I was having particular problems after skiing a boot for a few days.