The PH3 is not that old of a model. It sold for $1500 nine or so years ago and typically sells for $700-800 now. Other than playing with a couple different tubes, this unit should be mighty fine. I have really come to like the Mullard 6922 and 12AX7 tubes. With the PH3's somewhat forward presentation, a more balanced tube like the Mullard over the stock Sovtek would be a great benefit. If ARC replaces the tubes, you will pay more for the stock Sovteks than a nice pair of Mullards here on Audiogon.
Just because the PH3 has not been powered on for a few years, should mean nothing. I have had several ARC models much much longer with all orig parts and they worked fine. There's no need at all to spend a dime on replacing connectors either. The RCA connectors ARC uses should last a lifetime. It's not like this was a reviewer's model that saw many IC swaps on a daily basis. And even then, these connectors are fine.
The minute you spend anything on upgrading this unit, you will never see the cost come back to you. The PH3SE was the same unit with many improved parts. Why not try this PH3 with some different tubes, make sure it works mighty fine and then look into a more refined model. The PH5 is getting a lot of praise out there.
I had a number of ARC products updated and serviced. Being 3 minutes from ARC was a benefit as I did not have to worry about UPS or FedEx destroying anything. But this is always a concern...and shipping costs back & forth add up fast too. ARC service is first rate, but it adds up fast too. So my advice is try the unit, even with its stock tubes, and I think you will be mightily surprised.