You didn't provide the distance of the drive to work, or the kind of traffic, or other things but I would have to say if you want a TVR then get a TVR. For the 2500M the mechanicals are largely TR6 (as in engine, transmission, differential, suspension uprights, hubs) as well as things listed above like the gas cap, but if the car is not restored then it is a 30 year old car. Not that this should stop you if you want to (talk to Barry Rowe, TVRCCNA webmaster, who only drives TVRs daily between his 2 280i's and his 2500M). Damage is a concern if you're in an accident, so I wouldn't drive my 2500M to work daily in DC, but if I were still in Avondale, PA (my parents home) and driving the 3 miles into Kennett Square I'd do it. If you're not heart-set on the 2500M you could also go with a newer wedge (either from the US or the later ones from Canada) or even find the $ somehow to get an S1,S2,S3, or Griffith from Canada (which would put you in the same age range as a Miata). Parts availability for the TVR is not bad for a British car with the presence of TVR-specific companies in addition to the usual Triumph parts sources (I have an easier time finding parts for my 2500M than for my father's Sunbeam Alpine). You might also talk to Marshall Moore (TVRCCNA President) as he just purchased
Classic Motor Works and would have a good idea of the availablility of TVR-specific parts like A-arms, radiators, and body panels either through CMW, or the others like TVR NA, Exactly-TVR, and David Gerald Sportscars. Oh, and the TVR was tested at 26MPG by Road and Track in April 1974 ("25MPG & Over: 17 cars if all types, shapes, & sizes that will give you that kind of economy this year"), versus 29 for the 1990 Miata (
https://www.web-cars.com/math/miata_mileage.html)
As was said earlier, though, risk is a factor and depending on where and how far you drive a Miata may be a better choice. I'm graduating in 3 weeks and looking for a new daily now because I'll be working in either Richmond or DC and don't want to put the 2500M on the interstate every day.