I would use all new tubes if necessary. As it happens, I was able to get a pair of NOS military-grade RCA 6SL7 tubes for the drivers in a KT88 push-pull amp I have, which has made an improvement, as have also the higher grade PSVane power tubes.
Would you buy a tube amp if you were unable to use vintage tubes in it ?
Not available or too expensive.
Hmm.., I don't think I have a definitive answer for myself, but I would do my best to avoid such amps. There is no substitute for great tubes, I guess, especially if you value sophisticated sound.