I recommend 1/100 if you have a lot of HEIGHT shelf space.
* 1/100 is around 8-10 inches tall. It's a good size for the price.
* 1/144 is five to six inches tall, but IMO, has a tendency to fall apart even with the poly caps. Being so small, the stress and strain on the joints can't hold it together.
* 1/60...I've no idea...over a foot?
* High Grade = Snap together with NO internal structure (cheapest and easiest to make).
* Master Grade = Snap together WITH internal structure
* Perfect Grade = Snap together WITH internal structure and lots of extras (most expensive and most complex).
If you don't see HG, MG, or PG rating, that means that the Gundam kit was made in the 1970s or 1980s and is older and thus the engineering may not be as refined as the later HG, MG, and PG kits.
For buying, look all over online. Sometimes Toys R Us online as a 50-90% discount (you read right)/. Sometimes online COMIC book stores sell Gundam a lot cheaper than any online hobby store. Best bet is to do a YAHOO or GOGGLE under the Gundam name and see what pops up.
Even Hobby Easy or Lucky Model (see left ad bar) sells Gundams cheaper than HLJ and offers comparable customer service.