The reason why the space on the chip matters is because some of the chips have set aside memory for variables and others have (such as the 8 pin ones) share that memory with the program. More program, less memory for storage. Also it limits the amount of code you have.
I dont think the price difference between the 08 and 08M is very much, and I like the additional features on the 08M.
As for IR sensor, you can look at what the kavasar used, but I honestly dont know.
As to cost, that once again depends on what you want to do. Different sensors are different prices, chips are different prices, whether you make a pcb or use a premade board changes the price. Also there are all of the tools depending on what you have at your disposal.
Also, feel free to IM me at the aim username provided on my profile.