Ill just pile in agreeing with everyone here. I started my shop on my little 512 mainland island, and was able to grow at my own pace, till now I have 6,000 sq meters on the same sim. From the start I was so thrilled with my premium 512 (thats basically free if you pay a year at a time and factor in the stipend) - so I made 2 more premium alts who donated their 512 tier to group. Cheapest tier around :smileywink:
It has worked incredibly well for me - unlike some of my friends who rent I have never had a sim sold out from under me, I can build as I like, grow at my own pace, and have my friends sell stuff in my shop. I keep hearing customers express surprise that my shop is on mainland - I guess they expected it to be all ugly chaos, but it really isnt.
When looking for land I recommend picking an area that you like and just teleporting about using the map with land for sale ticked. My preferences is to have at least one (and preferably more) sides adjacent to linden protected land or water.