How old are you talking?
Pretty much anything newer than 486 will be ATX, which means it will all bolt together (other than SSD). As for cooling, some of the older ones had good cooling, some were terrible.
Not many older than 486 were towers of any decent size, most were sold as desktops. If you want older, you really need to consider server towers. These have the added benefit of better cooling, however, don't be surprised if you pay far more for shipping as you do for the box. Also, these are getting harder and harder to find. They were getting rare 10 years ago and unlike many things, we don't keep old computers, and most with a system like that would recycle it due to data issues, especially these days.
As for the oil bath...
Pointless. Yes, it's neat, right up until the day you have a problem and spend the day filling, emptying and testing over and over . Not to mention you will probably never get all the oil out of the parts. Oh, and make sure no dust gets in and makes it conductive... Even water cooling is all but pointless today except for extreme overclocks, and even then a decent all in one setup would work in most cases, and next gen chips and cooling solutions are only going to reduce that further.
While I like the old big boxes, they're bulky and unnecessary, short of overclocking, you can fit the same power into a shoebox that is just as quiet and weighs 1/10th as much.