So the end result is that you would have two 722s. I would not purchase a 722 for the following reasons:
You will be billed the same
Dish is going to bill you the same regardless of if you owned or leased the 722s. You are saving no money.
Early Termination Concerns are a wash
If you do lease however, Dish is probably going to ding you with a new two year commitment (or is it 3 years now?). So if you did purchase, you would have the ease of being able to drop Dish at your convenience without paying a an early termination fee (ETF). You would then have the burden of trying to sell your 722 to get the money back. If you look at it though, lets say you leased and then you had to back out and Dish dinged you with an ETF, the fee would probably be in and around what you would have purchased the 722 for anyway; so, with leasing you still would not be too far behind compared to purchasing. The ETF is prorated too, so if you terminated things late in the contract, the ETF fee will be lower.
Losing money on a purchase over time
Lets say you get a 722 for $300 (insert any dollar value here). After 2 years, you are out $300. With leasing, after two years, you are out zero dollars. You could have used that $300 for additional programming ... or whatever.
Warranty issues
Also, with leasing, if it dies ... Dish replaces it for the shipping cost ... with a purchase, you need to cover the whole replacement.
With a low receiver count ... leasing is the only way to go in my books ...
Now, if you wanted to keep your 322, then you would have to purchase, because I believe Dish only allows 4 leased "tuners". You can have more, you just need to purchase the rest.