Yep, the ultimate form of cooking!
I too started with one similar to yours. Wasn't enough very quickly. Pork roast, then two chickens, then a piglet. Not enough...
Then, I built my own! This was last year's Australia Day feast beast. Everything a boy cold desire - play with fire from 6am, and tend to it for the next 8 hours, and be fed cooling ales. Was the finest lamb I've ever had. Yum, yum, yum.
If you ever do go for a whole beast, look to stuff the abdomen as this will give a more uniform cooking mass to the animal so that the waist isn't cooked way before the chest. This will also keep the inside very moist and helps baste the beast from the inside out. The lamb I stuffed with garlic, rosemary, salt, heaps of bread, olive oil and red wine. Stitched up and cooked over some six hours. A pig I did was more sweetly stuffed with apples and walnuts too.
But I'm crazy that way,

. A neat little spit roaster like the one you listed Clive is a great culinary investment!