You should decide on what type of "portability" you want with this weapon. I own and Armalite AR-50 and love it. But, you'll be over 40lbs in weight on this rifle by the time you scope it out, add a bipod/monopod. They can be had for as little as $2,300 (search gunbroker.com) and range up to $2,600+. The AR-50 will provide the least recoil of any of the .50s on the market to date. The weight, and very effective muzzle brake, make this happen. My 7lb .308 hunting rifle produces more felt "kick" than the AR-50 does.
I've heard many good things about the other rifles discussed here including the Viper, Barrett and the AR-15 .50bmg uppers available. In the end...you have a 50...just decide how mobile you want to be with it.
Scopes. Many out there and many opinions to go along with them. On the inexpensive side...Hakko Super Sniper offered through SWFA.com. $299. I have 225 rounds through my AR-50 with zero "no pun intended" issues. Other will stanchly warn you away from these. The Leupolds, IORs, USOptics, ect are great scopes and carry warranties if they break. Check for "okay to use with .50bmg" when you purchase a scope with a warranty.
Depending on how deep your pockets are, consider the savings seen in each rifle and look at ammo. Unfortunately, ammo costs have risen substantially and unless you reload, you'll be paying over $3.00 a round for the surplus/basic ammo available. Something to consider if you plan on shooting the .50 a lot.