It's awesome when someone wants to get involved with Music, :3 .
Well, since your adults, I'm assuming he's relatively tall? A standard/full sized Cello handles people that are maybe 5 6" to 6 6" fairly well, and any taller and I would suggest a Bass :8 . But to be safe, he should be measured, although most adults can handle a Cello any who, but comfort is key
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As for price qualities... I would venture to say Glaesel, as they have wonderful quality instruments, but maybe abit pricey(for Cellos anyway). If it's just a fling to try it, you can try Merano, they sell mainly student instruments(which are still pretty good), great for beginners.
I've heard Florea combines great quality (maybe a 3 on a scale to 5) with a low price.
If absolutly serious, Luis & Clark make crazy good Carbon Fiber Cellos(super light, but sound magnificent), but are pretty pricey. Also, I've heard that some Chinese instruments are out of this world, but rumour has it that they quickly go bad
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I got my Viola as a gift, and it is Glaesel. Priced probably at 300-500$ :o .
Also, keep in mind, even a student Cello will cost quite abit, as it is a big instrument. Many people assume Violins are the most expensive(sometimes they are), but usually Cellos cost more.
(A Good Cello from Glaesel is like maybe $4-5000 and a Violin maybe $2-3000(haven't checked in awhile).
I would experiment with prices and such.
Lastly, if all that other stuff isn't what you want/he wants, then you can always ask to go to a Music centre or school that allows for Adult education, as they usually provide good quality instruments for rent.
I've been playing music since 4 and have been at the same institute and I've seen adults come and go, mainly looking to play Piano though. Music is for everybody, so go for it
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Cello is a very nice instrument, and less strenuous then Violin or Viola, if played right that is
. (Also, if he has any back issues he's gonna want a pillow to sit on, as I've seen my teacher do it frequently)
Good luck
(Try Music Box in denver, they seem to be good from what I could see.)