That sounds like 240 should be plenty. (If you had a newer vehicle with more electronics, I would probably go higher, but yours doesn't add much to the total draw).
The only thing you need to know is there are two specs for alternators that matter:
What is the amperage output at or near idle, &
What RPM is needed for max output.
Cheap generic
ebay alternators that say they are 300 amps, will put out less at idle than a stock 120 amp alt. They also won't give full output until well over 3K rpms. Well what RMP do you cruise at, (and if you haven't already verified this, then keep an eye on it next time you're out and know where it's going to be at most of the time)? A great alternator will do most at idle, and full output around 2K rpms. That's the difference between a $200 300 amp alt, and a $600 300 amp alt.
If any alternator seller can't, or won't give you those two specs, then move on. A great alternator seller like Mechman will give you the actual output curve for your exact alternator.