I'm also a Cat 5, and an older guy to boot; decided to start racing at 61. I got a PT via eBay and had it built into a rear wheel I already had. For longer intervals, such as the 20 minute TT intervals you mentioned, HR works pretty well. Where HR comes up short is in shorter intervals, where HR doesn't have time to react. Also, HR drift is real; last Saturday I did a series of longer intervals at 92% of FTP, and my HR noticeably increased with each interval, even though the power levels were the same.
If your finances are such that you'd have to choose between getting a Powertap or paying bills (and we've all been there), stick with HR. However, if it's a choice between a Powertap and a cool new wheelset, get the PT - it'll make you faster than the wheels would, assuming you've either done the homework to create a good training plan or have signed on with a good coach. It's a tool, not a talisman.
Another route to check into is a used wired SRM power meter. It's about the same cost as a PT, and doesn't require having a wheel built.