If you are on a real budget all that is needed is a $10 radio shack soldering iron, and $2 roll(the $6 spool has like 8X more solder) of rosin core solder

That is it



BTW why avoid irons under 40 watts?
My main used iron is a 30 watt and I've been soldering for over 20 years (had my cheapo radio shack for 16 or so years)
Only thing you need higher wattage for is batteries IMO, and if you are just doing batteries $20 300 watt iron would probably be best seeing it can solder those in less than 15 seconds