There are several inverters in the 300 watt range that are TRUE SINE WAVE.. The Samlex mentioned above (Which by the way I have only heard good things about).
Most all the TRUE SINE WAVE units I have seen are well built and have not generated many complaints.
MSW inverters you pay your money you take your chances.. But Most all the TSW's are good.
Mine is a Xantrex Prosine 2.0 9 years old and going strong.
One thing I will add: When I RE-Installed it (First one lost its mind) I upgraded the battery cables one size.
If the inverter you buy says "Use six ga wire" use 4ga (Which by the way is often less expensive since that is very common cable you can buy at auto stores).
DO not forget the proper fuse
From the inverter to the fuse TAPE the two wires side by side.. Then attach the positive to the fuse, and the other side of the fuse to battery positive.
Then attach the negative.