Bell siphons are one of the better siphons to use. If built correctly, they don't often clog or stop working properly. Normally the worst failure you can get in a bell siphon system is a permanent draining of your grow beds. Im not sure what you mean by siphon drain either. Personally I would leave out the lava rock I have never liked the stuff and hydroton works much better.
You should try a 12" depth for you medium in your grow beds, apparently many people have tried various grow bed depths, and 12" or 300mm is the best for almost every plant.
First I will warn again that you need to find out what kind of rock the gravel is!!!!!! Pea gravel only describes the size/shape.
Limestone and marble are not appropriate for aquaponics cause they will keep the pH too high.
The Place I get rock from is Just outside Orange County (but you didn't tell us the state so unless you are asking about Orange County Florida my info probably won't help you) pebble junction
I use 1/2" brown river rock which is mostly quartz type river pebbles.
If you are trying to set up aquaponics fish tanks to grow a fairly large number of edible fish in short order, then skip the gravel in the fish tanks. Gravel in fish tanks is only if you are trying to keep crawfish happy or provide breeding situations for particular fish or if you are setting up a display aquarium and plan to vacuum your gravel regularly. Gravel in the bottom of a fish tank in aquaponics situations usually just provides a place for fish poo to collect and become anaerobic and then if stirred up it can release toxic substances into the water.
Really heavy duty liner (The Firestone pondguard EPDM liner) can handle rocks and gravel just fine but I still don't recommend gravel in the fish pond except if it is an ornamental pond very lightly stocked and then the gravel is for planting the pond plants.