First question would be did u install the battery correctly? Second thing is that you could have forgotten to plug a cord back in when you put it back together or you could have even nocked something loose you didnt mean too.
erasing the hard drive shouldnt do anything. You could try to stick the start up disk in and reboot it from there. Update: i would try booting it from your other computer and if that doesnt work then there is something else wrong with the mac. Update: dude i just read what you posted in the first attempt and i would get a new hard drive cause i would guarantee you that that one is ruined because of the coffee.
its possible to change it if you can get the case. Id imagine you can purchase one online somewhere. To change it all you would have to do is take the battery out and unscrew all the screws that holds the case together. Make sure to know where all the screws came from. Also be very careful when taking the case off as there should be some type of tape or something that hooks from the inside to the case to see if it has been tampered with(a lot of electronics have something like this). There is a guide on here that you can follow to take apart the evo. If the scratches are small i would suggest to just leave them as its no really gonna matter in the long run.
most likely there is a problem with the hard drive. but it is possible it there could be a small defect on the mother board. this model is pretty old though so it could be just its age that is catching up with it.
you can take the front plate off by unsnapping the clips on the sides and top and bottom if you do this there are guides all over the internet on how to do it safely. once you get the top off you will be able to see if the wire connecting the dial to the motherboard. make sure it is completely connected or u can just replace the dial by ordering a new one and disconnecting the old one. you might have to do that, but it is also possible that part of the motherboard is burnt because of the water n being started after it. my iPod did almost the same thing n i just took it apart to see the inside.
depending on what all is smashed will depend on if a repair is possible. if the glass and or the lcd screen is smashed then that is simple to replace all u need to do is buy both of them which i don't think will cost much more then $100. if more then that is broken then it might be better to buy a new one, but its up to you.