what else is on the kebobs?? Onions, peppers, ?????? What sides are available? What is the gender and age of a majority of the group? makes a bit of difference, older group, mostly ladies, 20-30 YO males?
In addition: What cut of lamb? Are you buying bone in or out? Trimmed or untrimmed?
If you buy a cut like bone-in shoulder your waste is going to run very close to 50% -- and bone-in leg not much different. On the other hand, if you're buying bone out loin, it will be expensive as all get out but there won't be much waste.
It's a good idea to take a serious look at your butchering skills and your desire to do a lot of work before deciding whether you want to do your own butchering or just order cut meat from the butcher. While I'd do my own cutting, judging from the unsophisticated questions you probably shouldn't unless you're working with something very easy like the loin you can't afford.
Worth mentioning that you usually save very little by doing it yourself. Almost all of the price difference between fully cut and (untrimmed) packer is waste, not labor, (here's that word again) usually.
General rule of thumb is to allow somewhere between 4 and 10 oz meat per person, trimmed and ready to cook -- you can account for the 6 oz difference with shipscook's questions as well as a few others -- like: Is it lunch or dinner? Will it be the only protein? Sides? Etc.
In my experience people about 1/4 to 1/3 more weight off a skewer than they would as a solid piece.
If it's the primary protein for an adult dinner consisting primarily of coulbles, I'd say you're looking at an average range of 5-1/2 - 8 oz per person, which means something a little under but very close to 70#, dressed. Better 5# too much than too little.
You can get away with less meat per person, if you're doing a "kebab cuisine" type of dinner with lots of salads, grilled vegetables and a significant pilaf. If so, it might be a good idea to compliment the lamb with chicken (thigh) kebabs as well.
If you're ordering in quantity you can get long, flat, wide, "professional" type steel skewers for less than $2 per. You can probably get away with thirty, rotating in batches although fifty would be better. There's a limit to how many you can get on the grill at one time, so you're going to be working in batches anyway.
Just slide the cooked meat off the skewers onto previously dressed (disposable) platters -- and while the platters are making their way to the tables, you can reload and start firing the next batch.
The big skewers are not only a traditional part of kebab cuisine, and look a heck of a lot better, they make everything a lot easier. Beats the heck out of bamboo toothpicks with delusions of grandeur. Trust me.
If you want a crust outside with a mid-rare inside, you can cook on a hot grill. However, in the Med and mid-east, they're more often cooked more slowly over a lower fire. Also, you don't often see complicated threadings of vegetables on the skewers with the meat. A little onion and/or pepper -- maybe -- but rarely.
Mediterranean/kebab cuisine used to be one of my favorite parties to cater (despite there not being any money in it) and is still one of my favorite parties to throw. Don't forget the raki.