Re: Newbie. Nakamichi CD 45z-MB Quart pce 216-rce 269=Denon DCA 600 Questions.
My opinion on the head unit is that if your old one was good quality when it was made then it will probably sound just as good as the new one. I had an Alpine HU made in 1993, which I replaced last year with an Alpine CDA117, and the new one sounds no better than the old one, which was Alpine's top model in 1993. The BIG difference between the two is the functionality of the new CDA117. My old unit was top quality, but it had no processing ability, everything had to be done externally, such as crossover points etc. The new unit does so much, virtually everything can be controlled from it. But the sound is much the same.
50 watts RMS is pretty good to a component set, especially if you are planning to run subs to cut some low end to them. Not knowing the speakers you are using (ie power handling capacity in RMS), I would suggest that it would do a reasonable job.
As for your 6x9 mounting question, have a look in the Enclosure section on this forum for a thread I started recently called "8's & 6.5's in one enclose" or something like that. It is still on the first page anyway. There is some good info on there in regards to mounting full range speakers, covering position, and required volume etc. Worth reading before you go further.
Some of my gear is quite old. But if you are happy with the result then it will not matter at all. Personally, if that is what you have, and it suits the budget (not all of us have lots to spend) then I would install it carefully to get the best out of it. It should sound pretty good.
Alpine CDA 117E
Focal 5-3/8 splits
Focal 6.5 splits
Alpine 8" subs