Filtration Sanity Check Questions

Setup:
I've got a 46 gallon tank that's cycled nicely for 12 weeks. Gravel
Substrate, Plastic Plants, 6" bubble bar, and Penguin 170 Power filter.
25% water change once a week, 78-80 F temp, South American Flooded
Forest Biotope.

I've been adding in fish and invertebrates for the last six weeks- and
I'm pretty much near the limit of bioload I should have in the tank. (44
inches of adult size animals. Most of them are currently juveniles at
less than 1/2 of their full size.) However, as I'm adding fish, I've
noticed a steady decline in the ability of the biological and mechanical
filtering to keep up.

Everyone's healthy and happy so far, but I don't want to run into
problems down the road, and I'd like to add more filtration- if nothing
else, for a backup system when I need to clean out the penguin, etc.

Other parameters:
I can't have clutter and noise where the tank is placed, have less than
$100 US to spend, want parts I can rinse out and put back in (unlike the
Penguin 170).

Questions:
After doing some homework on the net, I've settled on getting a
submerged internal filter, probably a Fluval 4+. (I especially like the
idea that I can add a peat filter to it to help out the pH, but that's a
minor concern.)

Does this sound like a good choice, given these parameters?
Is the Eheim Aquaball worth the extra dough over the Fluval? If so, why?
Is the 4+ overkill? Since I already have the Penguin 170, would a 3+ do?

Thanks in advance for any on-topic opinions.