Hmmmm. The 1975 Ford F-100 was really good truck 30 years ago.
Same goes for the Maverick 250. It's not just the years, it's the miles, too.
Anybody who'll quote you a price to fix it SIGHT UNSEEN should probably be avoided.
The years will have caught up with all the electrolytic capacitors in the thing. Any that are still original really have to be replaced if you plan to use it.
The relays are a wear-and-tear item. If it had an odometer, you could judge how much wear to expect. When one of the two relays goes bad in that one, you just have to replace it.
Tubes are an issue. If they aren't right, the amplifier won't be, either. Changing tubes will often change the tuning behavior of both sets of "tune" and "load" controls. Small adjustments to the coil strung between the controls may be necessary to compensate for internal differences between the old tubes and replacements. The best final tube to use is also the most rare and expensive. The 6MJ6 tube has a "black" plate cap on the top, with the glass-to-metal seal visible around the shank of the cap. The cheaper, more common 6LQ6 will work, but has thinner glass and will not take abuse nearly as well.
The 6KV6A tube is the best driver to use, but (surprise) the rarest and most expensive, too. A handful of other types will drop in and work, but all have lower power ratings. Tends to make their service life shorter, as a result.
Somebody who knows all the stuff that SHOULD be going bad in the thing will prepare a 'laundry list' of all the details that are needed, after looking it over. Until then, who knows what it will cost? Nobody.
Even if you find that '75 F-100 parked in a barn under a tarp with 5000 original miles, it will need a long list of gaskets and seals replaced before you hop in and drive it to California. Not to mention tires.
If someone has already caught up some of the maintenance issues over the last 30 years, you won't have to cover them all. On the other hand, if it's "MINT", and all that stuff is original, you'll need to spend a wad of money before you can rely on it.
One last thought. If this Maverick is the ORIGINAL version with THREE relays in it, the price just gets higher. D&A figured out that using just two relays was a lot more reliable, and cheaper to produce. ALL later versions, through the last "MDX" version were built with the 2-relay design. If you want to fix up a 3-relay Maverick, getting it updated to the "2-relay" setup will get you more miles before the next breakdown.
Whew! And that's just the short version.
73