What type of solder to use with unregulated 25W soldering iron?
Any common leaded solder (60/40 or 63/37) with rosin flux core will do. Iron wattage does not dictate solder type. Wattage mainly dictates work piece size. Use as big a chisel tip as you can possibly get away with. I recommend 2.4mm for what you described, maybe even 4mm. Conical tips are terrible...stay away from them.