'Best' is really difficult. Someone who has X on their car will claim X is the best because it's working and running their engine. Someone will say X is the best because their mate reckons it's the best. Someone will say X is the best because it's their favourite tuners favourite ECU. Someone will say X is the best because it has the longest featurelist. Someone will say X is the best because it's cheapest. Someone will say X is the best because it's in a pretty purple anodised case. Etc etc etc.

It's difficult to get a factual opinion on what's 'best' as 99% of people don't really know what makes a great ECU....

ECU prices range from £300-2500 for the aftermarket typically. Add £300-400 on top for a loom. Then you have things like fuel pumps, injectors, sensors, manifolds, etc. Can you do it? That depends on how much you know about engine and how to tune them. Anyone (should be able to) learn.