Have you talked it over with Axminster? They know their machines better than any of us, and they're generally honest brokers when it comes to giving unbiased advice.
You don't want a power feed that's too big any more than you want one that's too small, for example the mounting plate on a bigger one might interfere with the machine fences or guarding. Also think about how easy it is to dismount and re-fit, as it can be a struggle for one person to move them on and off the spindle moulder.
Good luck!
Edit, just seen your follow on post so ignore this!