The way I see it, you have two options:
1. Get a TQ now and use the pad as your under insulation
2. Get the UQ now and just use your SB partially unzipped as a TQ
I would personally go that route. I've been using a sleeing bag as my TQ for over a year now, and there is no draw back except that it weighs a bit more than the TQ would. Of course, it's also free. I think you would be more comfortable sleeping in the hammock with a sleeping bag and an UQ than with a pad and a TQ.
As Slo mentioned, I would skimp on the TQ before the UQ. It is easy to suppliment a TQ with extra clothes, a liner, or even a second quilt. For example, I know people who own a 40* and 20* burrow and simply use both if the temp is expected to be below 20*. Supplimenting an UQ is more difficult, but venting it in warmer weather is very easy. HYOH.
"The Road goes ever on and on,
Down from the door where it began.
Now far ahead the Road has gone,
And I must follow, if I can.
Pursuing it with eager feet
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say."
~Bilbo Baggins - LotR