So I am interested in buying some older ADCOM Equitment , yes I like the style and looks, very plain.  Any advice on what series / model #'s I should look for? 



Pre amp?

CD player?

Thank you in advance.

I owned a GFA-555 for 30+ years.
I worked as good for all those years until I sold it a year ago.
Nelson Pass did a great job on those amps.
I have no experience with other Adcom equipment, but suspect it to be equally good.

I own a 545v2, also a Nelson Pass.. sounds pretty good.. especially for about $250.. I keep as a back up for when my main amp is being switched or fixed.. I never feel I'm missing that much when in use.. My main amps float n the $6000- $8000 range new
I've owned various Adcom items and they were always nice sounding and reliable...I went for the tuner-preamp with a 535 when they first came out, later a balanced preamp (can't remember which one) used with different power amps, and more 535s (used and new, including the 535II) both for studio and otherwise. Sold 'em all eventually except a 5300 that's used for remote (deck) speaker duties...that amp sounds great and was seriously inexpensive used. Nice stuff, but watch for oldness issues...leaking caps, hair loss...
In the past I've used a GFA5802 to drive Martin Logan Ascent ES hybrids. All MosFet heavy bias AB amp. A fair amount of heat but no fans. I had to go to a Pass X350 to upgrade. The Adcom is still on active duty driving a pair of JBL 4430s.

It's a pretty good amp for the money.
I had a GFA 555II for a couple years. Bought it used. I never had tuff speakers to drive,so that thing never broke a sweat. I think I sold it for what I paid for it.
The GFA 535 amp was reputed to be the sweetest sounding of the amplifiers, if you don't need the power of the 555.  I agree with Wolf, you should watch for issues related to age.
5802 is a great sounding amplifier, and a pretty good home heating device to boot.

I re-capped mine for safety about 6 years ago.
Had a 5500. A big, ballsy, over the top amp, let down by graininess in the mid and upper frequencies.  I sent it to Stan Warren to be upgraded and bought a Sunfire Stereo 300 to use while waiting.  The Sunfire was not the over the bombastic amp the Adcom was, but crapped all over it in lack of grain, black level, and simply disappearing.  The sunfire was far better than the Stan Warren modded 5500, so I sold the Adcom and kept the Sunfire.  15+ years later, I still have the Sunfire (rebuilt last year).  

I also used a GFP-750 for years, great preamp that I just replaced with a Schiit Freya S, which images better, but the GFP-750 was bit better in being perfectly transparent.  The passive mode in the GFP-750 is the best passive I have heard.  

Have had other Adcoms, GTP-500II, GTP-400, GFA-5300.  All were good units that performed above their pricepoint.  
My main amp is tubed Conrad-johnson Premier 11a, but I have an Adcom 555II I used before C-J. I use it as a backup, I have had it 20 years & NO TROUBLE. Nelson Pass does good work! I love my C-J, but the 555II Is an excellent alternative. IMHO Amps are important, but PreAmps really set the tone. My preamp is an Audible Illusion L3B (brand new, but there are several used on the market)