You're size requirements will be difficult to meet.
Do a google search for a Mean Well DR-UPS40. This unit provides the charge controller functionallity. You have to supply the back up batteries and and a 24VDC supply. The DC supply must be sized to be able to run your system + another 2A to be used for battery charging.
The DR-UPS40 runs off the 24VDC supply, so it doesn't care if your local power is 230VAC.
It's a good starting point and at about $60 you can't go wrong. You won't get too much custom built at this price.
Gary d