In reply to Joshmackintosh:
Well Josh, I'm one of those people who have criticised the Everest scene, commercial expeditions, bottled oxygen and fixed ropes on here for years. I still do, for many provable reasons beyond personal style preferences.
But you asked a specific question and in the spirit of internet climbing forums, and Christmas (which I also f*#@king hate) I will offer you a slightly alternative response.
A lot of the advice on here is well-meaning, and traditional - but wrong. I say this not because I've been up Everest, I haven't, it looks awful, but I've climbed a bunch of other mountains, some high ones, have numerous Everest-guide friends, and have seen many people come in, go through, and out, or not, the process.
If you try and do a lot of Scottish climbing you will end up watching it rain from the car, tromping around in a whiteout and not getting much climbing done, and certainly not climbing any high mountains - which is ultimately what you want to do. To climb Everest you don't need be able to run it out 20ft above your dodgy tied-off screw on some obscure grade VI in a sleet storm.
Nor do you need to be able to climb hard alpine routes, something that would again take years of part-seasons and much failure and weather-watching. These are the traditional paths of British climbers to high mountains, but they are quite inefficient and no guarantee of success - in some cases quite the opposite. Times have changed. Everest is now something else, not really climbing.
You need to be physically very fit (but mostly legs and lungs, no excess muscle mass) and mentally up to it - both to handle the stress, the fear and the boredom and uncertainty. The mental side partly depends on your starting point, your personal history, personality and character now, but whatever, it needs to be built on by doing at least a few high mountain climbs.
Learn how to stay healthy on a 6-week expedition, many fit and experienced climbers have spent their $50,000 only to get a debilitating chest infection at BC and go home early.
Know what you need to eat up there, what makes you vomit.
Know what gear works for you, what fogs your goggles and what doesn't. What crampons don't fit on the only boots that are warm enough for you. What mat is best to sleep on, what gloves don't incapacitate you at 3am in -20C. This will inform you what you need to take that no company may provide.
You need to be able to crampon across an icy slope of 45 degrees unroped, when tired after a 10hr ascent, just in case you have to, without slipping and falling 1000m to your death. Forget WI climbing and increasing your sportclimbing grades - these are no help. So long as you can tie in safely and quickly every time (tired, in the dark, in gloves) and move safely and efficiently up a Diff, or Scottish II, you will be fine.
Basic alpine routes are good, such as normal routes in the Valais, and by all means do climb Mont Blanc, preferably unguided once you've got the basic skills and done a few lower peaks to acclimatise properly. But don't take too long on this early phase - you just need to get proficient at the basics.
Once you get to that level, don't worry about getting any better technically. Focus on going high, forget Elbrus, so go to the Andes (Sajama, Illimani, Huascaran) for at least three weeks, at least once, and climb a few 6000ers, either guided or not, and probably do Aconcagua to see the kind of clusterf#@ks you may encounter on Everest, and to get to almost 7000m.
Don't worry about steep, famous or technical peaks, or style. Just climb high safely and efficiently, see how you feel, and preferably do as much of it looking after yourself as you can, safely. Have some long days, get a bit worked, learn how to pace yourself and what YOU need to do to acclimatise well, which may be notably different from someone else, or the company brochure. Most itineraries are made for business, not health.
If you can climb such things - just one or two trips in two years, not five trips in a decade - and haven't lost interest then you can probably go climb Everest, because realistically nowadays you will be with a company that provides at least one personal Sherpa and enough bottled O2. Don't try and skimp or play hard man and go with less than this. Most climbers who try that fail, some fall back on others for help - and quite a few die. And if you've followed what I've said above you won't have the experience to safely climb it without bottled O2 anyway, and that was not your goal in the first place.
Decent companies will tell you to climb another, lower 8000er first. Mostly this is good advice, but plenty have summited Everest without doing this, and some companies just want the extra business. But it also gives you experience up high around 7500-8000m and experience with the scheduling and systems inherent in such climbs - bottled O2, ladders, summit schedules etc - and if you do it with the company you will climb Everest with, you get to know how they work, maybe with the same leader/guides and Sherpas. Such familiarity is not strictly necessary but will increase your chances in the end, as being familiar with their setup, systems and people removes unknowns for you, which means fewer chances to mess things up. There's already enough.
After all that it really comes down to what you can, and want to, afford - western guide? extra O2? comfy BC? - but how much of that you will need you will only know after doing something like all the above.