I think converting an old road bike to a TT bike is a good financial decision. Just don't make the mistake of keeping a standard seat-post. Get one that can be positioned to give you a steep seat angle so when you are stretched out on the aero-bars, your body can move forward. I see a lot of people riding aero bars and a regular seat post. They've lost a considerable amount of efficiency by doing so.
Also, get a stem that has a fair amount of negative angle to it. This will also allow your upper body to move down, giving you a more comfortable ride (believe it or not!) in the TT position. Move forward, move down, and your body has essentially moved around the bottom bracket. Just like on a TT bike.
Remember, the biggest aerodynamic concern is your body position. Never mind the aero wheels, aero tubing, aero helmet. All of those are great when you have finally dialed in your ride position.