Hello and welcome to AK! I have an A500 which is the factory built amplifier. Your A50K is the kit version.
This amplifier has a really lovely phono section. If you are a vinyl fan, this is a very nice sounding amp to delve deep into an LP obsession. However depending upon the size of your space you will want to make sure your speakers are efficient. I have found my HK just does not have a ton of power. It is rated at 25 watts and it feels like it is a quiet 25 watts. Speakers with at least 90DB efficient will sound nice. I have a pair of Snell speakers that pair well with the amp.
I did not do any work on my unit myself, so I cannot help with wisdom about DIY updates. However I would check out this thread:
http://audiokarma.org/forums/index....rt-on-just-aquired-harman-kardon-a500.249364/
I have not contacted Jim McShane myself, but I understand if you reach out to him he can help with kits for updating these units.
http://www.mcshanedesign.net
Also here is another resource for the HK units:
http://shermanr.web.prw.net/Page_02.htm
Good luck with your restore! And please post on any progress. The HK amplifiers are well-built units that have wonderful sounding phono stages. Anything you can add to the knowledge base for the A500/A50K would be appreciated.
Patrick