I know of no tablet, with any operating system, which has a sunlight readable LCD and which costs much below £2k. There may be some coming soon; I think Motion have some new model out.
As regards building an "EFIS" product using a tablet, this has been done, using one of the off the shelf AHRS products, and "a bit of software"
I saw that at various aviation shows. Pop over to Friedrichshafen (EDNY) in April and look in the homebuilts section.
Remember there are various issues with AHRS running alone. Unlike a self erecting mechanical gyro, it cannot tell up or down so if you interrupt power during flight, you lose it "for ever". The commercial AHRS products seem to fix this erection issue by introducing external info which suggests the plane is actually upright and level e.g. IAS constant and heading constant and altitude constant (fine so long as you are not flying upside down
), or using GPS data (GS constant, track constant, altitude constant). I believe Aspen use the former method (and your pitot/static system must be working) and Garmin use the latter method.