how realistic a ceiling crawl or just a cling actually is with the existing gear available..
It is indeed very realistic, however, it is as dangerous as anything done without proper gear. Standard crashpads that you see in climbing/bouldering gym are just fine for this.
I would imagine that these two would be a combination of body conditioning and the gear available
Correct! Add experience and practice/technique to that list. For some it takes years to get to this point, for some it is just a matter of a few weeks. But, to be honest, I haven't come across people who managed to climb roofs and overhangs in a matter of a weeks. Months - yes. Not to undermine anything but I touch-based this topic about time-spent because of your explicit admittance about having limited climbing experience. A lot of effort goes into developing technique to climb as well as to fall safely. It is one of the trickiest learning curves.
so would be interested in exactly how much training/experience would be involved?
The only possible answer is 'adequate effort/experience'. This can not be generalized as it may (in fact, will) vary from person to person depending upon a lot of aspects.
does it require only a specific surface, specific shoes or other gear and also is there any way it could be done on an inside ceiling?
An overall advice: I'd start with attempting various climbing routes in a nearby climbing gym. At a certain point, I'll figure out that my shoes aren't good enough, and thats when I'd invest into a good pair of shoes. Eventually, under the proper guidance, when I'll attempt tougher routes involving dynamic moves, overhangs, roof sections - I'd precisely know what gear I need and how to use it with proper technique.
Now if I try to quantify this for my case, it would take years, and still I may not get to the point where I can cling/crawl on to roofs! But, what I would have gathered over this period would be experience and technique.
Speaking of climbing, what I might be able to achieve in an year, I have seen others to that in months.