Do people not use horse vacuums here? There is a fairly common "equine-specific" model and quite a few barns have built in ones but lots of people just use heavy duty shop vacuums. Most horses are okay with it with a bit of work and one bonus is they usually get better to clip, too.
The less expensive option is to buy a "Henry" vacuum cleaner (on ebay), then go onto their web site and get the special long hose and the grooming brush attachments, mine cost about fifty quid all together.