I fitted a Heatseeker HS20 cam to my XF and it went somewhat better than the std ED - ie. was pretty tough off the line and effortlessly reached 150 - 160kph (that's as far as I ever took it because at that point I hadn't got the suspension set up to cope with those speeds). It wasn't until I got an XR6 cam into my ED that it started to feel similar in outright power terms to the XF with that HS20 cam (having the Webber carby properly sorted was a major thing too).
The changeover took me a full day and involved taking off the rocker cover (and rockers etc. of course), removing the radiator, removing the front bumper, removing the front upper cross piece and then removing the harmonic ballancer and timing cover.
I also replaced the lifters for which I bought a good quality lifter puller - which was a wise choice as pretty much all of them took a fair bit of fiddling to get out.
The only thing that pissed me off royally was that altho I tried really hard to get the timing cover aligned properly - as in shaft seal centered as well as possible (I replaced the shaft seal in the timing cover with a new one) - the damned thing then leaked oil consistently from that front seal - only a few drops on the driveway per night but it was still a pisser as she'd been dry as a bone until then.
In short - a competent mechanic should be able to do the changeover in a day - but definitely budget for a full day's work to do it.
As far as tracking down cams it's probably worth checking Crow Cams website (or just call them) or try finding a Heatseeker site. Also try calling Wade Cams.
IT'LL GO OR IT'LL BLOW!!!