I may have got this wrong but in my head there are broadly two well-trodden paths.
BIAB is typically mashing the grain in some sort of insulated vessel. This needs a simple water boiler like a cafe. To heat a quantity of water, the mashing is done in another vessel and then returned for boiling. CWRW666's advice is sound. A cooler box is a good choice. Many of us started there.
Malt tube type brewing. This is where the grain is on a stainless tube or bucket. Inside the kettle, the temp is accurately controlled and the liquid is pumped around d through the grains. This is the fundamental design of a "all in one" brewing kettle. The grains are removed after mash for the boil.
So if you are planning to build one. It is very do-able now. You need a basic cafe boiler. Rip out the thermostat, replace with a digital controller.
Make or buy a snug fitting stainless (mesh bottom) basket.
Add a food safe pump, connect it to pump from the bottom tap into the top of the grain basket.
I would also consider a power controller. Full power for boiling, but half for mashing. Or two elements.
These were popular builds a few years ago before the influx of brewzilla/grainfather/brewmonk etc.
But to be perfectly honest, I still think it would be cheaper to buy.
At the moment you appear to be in a twilight of both ideas. Perhaps it might be better to find what is available to you, easily and cheaply is Spain, and we can help you from there?