should i look for the newer model or the old one is fine?
as noted above there is no correlation between the KX amp your friend has and the new series of Kicker KX amps. At present time, there is not a 11KX###.2 stereo amp that offers similar power to that 04KX550.2 stereo amp. The ZX450.2 is the closest, but as suggested, I would go with a Class-D due to their size and efficiancy. That 04KX and the current line of ZX amps are all Class-A/B. A Class-D amps can be as much as 60% more efficiant. This means its drawing far less battery amp/hours as the Class-A/B, while producing the same wattage to the speaker. if its in your budget, there are clear advantages to going with a Class-D amp like the IX-500.4. But, if you are trying keep the budget on the low side, then its hard to pass on a $50 amp thats going to deliver 200W rms. Or, go with the new Class-D KX400.4 or KXM400.4 (when it rolls out soon) and run it as a 2 chnl amp and get the same 200W.
Sorry, did not realize you had a
private sale lined up. Since you linked to an online retailer, I thought you where looking online to purchase. If your friends amp works, then take it for $50 and rock it till it dies. The KX550.2 will be solid power for them. Just run it in High-pass tuned to about 125hz and you should be good to go.
We use a lot of non-marine amps when there isnt a marine amp thats fits the needs to of the build or the budget. In a fresh water trailer boat, you should not have any issues as long as the compartment is allowed to vent when the boat is stored and there are now drip or drain hazards above where you plan to install the amp.