Provided you have no preferences on what you keep, your plan sounds good. I'd sell the Glock as it will be cheaper to buy another later than it would the USP tactical. The reduction of income will make it hard to buy a high-dollar gun like the USP/tactical in the future, and since you own it now, you might as well keep it.
I would probably go the same route as you and buy a 9mm, though probably something other than the HK just to have more variety. Something like a Springfield XD, CZ-75 or a Browning HP would be pletny fun to shoot and far cheaper than the HK. But if you like the USP there's no reason to not to get another.
Another alternative if you're income is being reduced by 4/5 (whoa!), is to buy a cheap reloading set-up. Everything you really need (none of the super-dolomite set-ups, but just basic stuff) is dirt cheap and you can find it used even. About 80% of the cost of .45ACP is for the brass. I don't know how the USP is on brass, so reloading may not be an option, but it will cut costs and leave you all sorts of money in the bank. Money in the bank is always a good thing, especially with such a drastic change as your's.
Ross