I have been using Rena airpumps for 25 years or so now and I found them very reliable. The new breed are ultra quiet and efficient. If you wish to expand your stock/tanks at a later date the I would be inclined to purchase one a little larger than you actually need, that way you'll avoid buying a new one later on. I recommend you buy a Rena air 300. and connect this to a bank of gangvalves and leave one outlet free to bleed off excess air when its not required. When you expand then you can connect another airline to this and regulate as normal.
Regards
BigC http/www.aquariumsuperstore.co.uk/mall/renaairpumps1.asp