I can't speak specifically for Elbrus, haven't been there. Many guide services recommend doubles because no one loses toes from having too warm a boot. Also, doubles are much easier to run for a rental service since they don't break in as much and are more like ski boots. They fit a wider range of people. Perhaps ask your guide what s/he will be wearing.
On Rainier, for instance, I've seen loads of clients wearing Scarpa Invernos whilst their guides were Nepal Evos.
But in absence of real detail, go with the company recommendation. You are paying for their expertise, so take their word over some stranger on the internet.
Evos are indeed kinda heavy, but not terrible. I liked how they walk better than their nearest competitor (Scarpa Mont Blanc), the pivot point felt more natural. But it depends on your toe-to-foot length ratio. You can wear them on an approach, I have many times. Try to find a spot to dry them out before the real climb, though, they might get a bit damp.
You might indeed need double boots for Elbrus (I don't know), but my point was more that if you are buying a boot, and you are only buying one pair, then it should be a single insulated leather. You'll get more miles and days out of a Nepal/Mont Blanc than you will out of most any other boot.
You still might need to rent doubles from time to time, and sometimes you may wish for lighter ones. But they are as close to a one-for-all boot as you'll get.
I also have a set of neoprene overboots that I can throw into the mix. I've worn them with my summer boots when it got too cold. And also with my EVOs during the winter. Don't work well unless you're wearing crampons or snowshoes, as they don't have any tread. But that's ok since you only need them when its very cold.
http://www.backpacker.com/gear-zone-gear-review-crescent-moon-over-shoe-booties/gear/15159Eventually, you'll want trail runners, light hikers, summer mountaineering boots, high alpine, and double boots, not to mention climbing shoes. But one thing at a time.
Hey, if you're a size 43 2/3, I have a nearly new pair of Asolo Eiger GV that might work out for ya!
http://www.geartrade.com/item/329759/asolo-eiger-gv-excellent-condition-300