For monitoring 1.2GHz transmissions (power out, SWR) I can highly recommend the Diamond SX-1000. It's a nice meter for the price. Look it up online; one of the many nice features is 2 radio inputs and the ability to switch between them.
For a 2.4G signal source--search for the board designed by G0MRF. It's a nice signal generator to make sure the 2.4GHz receiving system is working. I bought one of these a while ago and have appreciated having it handy many times.
Hope that info helps! Glad to see you moving towards L and S band ops :)
Mark L. Hammond [N8MH]
If you happen to have a Bird wattmeter, it works very well on mode L using a 400-1000 MHz slug. It will be within 5% or so of a "proper" 1.2 GHz slug, which is plenty good enough for amateur use. I have a Bird which I normally only use for development work, and I just moved its storage location to the shack with the coupler on the back of the amp.