First non-mountain bike

Disclaimer - I made no attempt to read through all of the "my first bike.." and road vs tt bike posts. However, I did read quite a few of them. From what it seems, everyones situation is a little bit different and so I thought I'd post mine, and see how everyone weighed in.

I'm planning on buying a used road bike or TT bike.  I've seen some "good" deals pop up and am wondering what to get. For what its worth most of my rides will be alone or only with 1 other person because my schedule is pretty hectic and sometimes inflexible.  My main concerns are 1) buying more bike than I can handle, and 2) spending good money on a bike only to find out I now want/need to upgrade to a nicer bike. Point 2 mainly reflects the idea that if I use a bike, sell to gain $ towards next bike shortly after buying it and then buy next bike, my expenditure will be more, or perhaps significantly more than buying the nicer bike in the first place. The idea here is to decide between a TT and a road bike. I haven't mentioned this to anyone yet but is buying a TT bike and buying regular road bike handle bars to switch on and off of it a possibility? Just thinking out loud there...

Before I buy the used bikes I'm going to the dealers and seeing how each of the bikes fit on me etc. I'm also looking to upgrade my scuba gear so buying a more expensive bike will mean delaying scuba purchase, but I'd rather do whats better "right" than just buy all my toys at once! Please let me know your thoughts on the overall bike to purchase (TT seems to be a bit more expensive for the "minimum" purchase than the road bike does).

Thanks!