Not sure what you mean in the first part Max, but will give my opinion for the ladder part.
Usually if I am in need of a motor modded, though it seems that the mfg.'s have decided to just do their own now. I am looking for any number of things or combination thereof. Hp, Torque, reliability, powerband, fuel consumption are factors that I will look at and then if I can get some wheel time see if the motor fits my driving. Am I faster or slower with it. Out here in the West many tracks tend to be slick outdoors at somepoint and should be taken in to account because a super high Hp motor on that type of track may not be the best. Not that its a bad thing to have it, but not always needed. Its like taking the baddest on road motor where traction is usually high and the track can take the Hp, but loose the high traction and the motor can go from the best thing since sliced bread to the worst thing out there. Different applications and track types will require different motors to a point.
Another thing I will take in to account is the reputation of that modder. More so on the customer service side of that. Can I easily get in contact with them if needed, tech. support if I need it? Those sort of things. A big plus is if I can run accross them from time to time at one of the tracks I go to? Sometimes that can be a big help on race day. Lastly for me would be price for the service. If its really worth it, price usually comes in last.
One thing that my be a factor for many would be, did it win on Sun. and who was the driver? Happens alot. Sorry for the length Max.