several points spring to mind. and please keep in mind i am not trying to be difficult or put you off the project, what follows is my attempt to help you build a good incubator.
first off you need to consider if you want a top end incubator with exceptional hatch rate sucess (this will cost money) or can you settle for a less exspensive slightly lower hatch rate.
when considering this be realistic. think about what your likely to be hatching most of, for example we have incubators here that are reserved for hatching high end eggs such as those from rare breed birds of prey or parrots, theese incubators cost a fortune to build and run, but the value of each egg is very high so it ballances out.
we have more normal incubators that we use for general hatching that arnt as complicated or exspensive to build but the hatch rate is slightly lower.
incubating different species together and getting optimal hatch rates is just about impossiable to achieve. if you settle on doing this then you will have to except lower sucess rates of hatch. for example hatching ducks and chickens eggs together is common but you have to favour one or the other with the settings, for examples duck eggs do best generaly with a 23 min cooling period where the heat is allowed to decline slowly for 23mins the turner should also be off dureing this time as the membranes in the egg start to contract everso slightly but can lead to internal blood vessel rupture (you cant see this without a microscope). chickens on the other hand do best with 3 7min rest periods each day, but the auto turner can be left on.
some duck species do best with 5 or 7 complete turns a day while chickend do better with 9 or 11 turns. also humidity (optimal) is different for both. geese are again a whole different ball game and realy do way better on there own with a exclusive setup (last few years we hatched around 20,000 geese a year). the point i am trying to make is decide now what your main goal is. how advanced do you want the incubator? in a high end incubator you also need to carefuly consider airflow.
if your main aim is just an 'incubator' then thats pretty easy and cheap. also you need to decide how many eggs you want the incubator to hold this is realy step one as it has alot of implications on the design.
have a long think on what you want and i will help you where i can