I did similar a couple of years ago. Went for a PX Stealth frame in the sales. Stuck mostly 105 group on there. Used some old wheels with a aerojacket cover on the back. Probably cost me less than a grand. Though then there’s the aero helmet, skin suit, shoe covers… etc.
Gradually upgraded the bike with a P2M power meter, Corima disc wheel, PX 50mm carbon front wheel, new Zipp bar assembly, and now thinking about Di2, so it’s now a bit more than a grand 🙂
As far as the frame goes, provided it can accommodate the correct position I think there’s better places to spend your cash.
Got a pointy hat yet? That’ll give you more time/£ than a new bike.
While this is true, assuming you can get a good TT position on the road bike, the setup changes are a PITA and it’s much nicer to have a dedicated bike for it. Also tri bars on a road bike are just wrong!