Welcome to Fishlore luman!!

Seeing as the tank is empty you can work out the exact volume by pouring in an exact amount of water, say 20lts. Measure how much that fills the tank in cms from the base glass. Then all you need to do is multiply the figure you have in cms to the full tank depth. If 20lts fills the tank, say 5cms, you will need to multiply 5cm 18 times to get to the full tank depth. 18 multiplied by 20 is 360lts of water to fill the tank. Remember that is just hypothetical, not how much your tank actually holds. I get the tanks volume to be about 300lts just off the top of my head.
For filtration I would go for 2 x canister filters that are suited to tanks of about 250-400lts. When it comes to filtration there are better minds on this forum than mine so hopefully one of them will be along shortly to help you out. At least you can work out your tanks exact volume so that you can start to work on your filter options yourself.
