When I first started getting away from the crowds on piste, I used devices called BCA Alpine Trekkers which make downhill bindings tour compatible with regular boots and they are a good compromise if you don't want to go the full hog with touring hardware and the expense of dedicated kit. After all, one guy in the States used Alpine Trekkers to ski 53 of Colorado's 54 14,000 foot peaks!
Downside is the weight which allied to the fact you'll be using heavy downhill skis and boots mean some extra resilience and fitness is needed. Upside is you can go playing out on the odd occasion we get snow in the UK on a shoestring and with your regular downhill kit – just source a pair of second hand skins (if you can!) and you're away!
Soon enough, if you persevere with this kind of rig you'll hanker after the pukka gear. (I did!!) In fact I got so much into ski mountaineering and touring, I set up a small section of our shop with product and started running alpine courses to get more people into it!!! In fact I can bore the pants off anyone with my enthusiasm for the game...
By the way, on the subject of skinning (walking) with ankle and power-strap loose; yes, this'll give a downhill boot more flexibility but it's also a sure fire way to get some mega blisters after a few miles. Take it easy - I tramped the entire Haute Route with a raw heel and found out the hard way! As soon as you have the chance, buy a specific pair of touring or freeride boots – as a rule of thumb, the stiffer the boot the better it'll ski downhill but the worse it'll be for walking all day in!
I hear some people suggesting Dynafit bindings but I have found that although very light, the release facility is perhaps not the best for the average joe when using them in anger and I prefer the evergreen Fritschi Diamir's in whatever format takes your preference. Dynafit's have a great following amongst the professional fraternity such as guides etc.
The subject of ski mountaineering as a pose to ski touring is a vast one and depends on your definition of mountaineering. The hardcore climbers are merely using the skis as a means of transport to their main objective - a technical route and the kit just needs to be functional enough to ascend and then survive a descent where style and maybe even enjoyment are not the major issues. For them, second hand, more traditional kit available more cheaply will probably serve quite adequately.
Guys like me and my mates work hard on the ascents and want to savour the descents in all conditions, so the current crop of big mountain skis together with midweight, strong and safe freeride bindings provide awesome performance. Yep, they're not cheap, but how much did it cost you to be out there in the first place? Exactly...
But best advice of all. STAY SAFE. Carry the safety kit and know how to use it. Take local advice or hire a guide between a few mates – their knowledge of the backcountry where they work is priceless. Just 'cos we CAN get to the backcountry doesn't always mean we SHOULD be there...