Yes too big can cause several issues, rusting out the heat exchanger, vent, inducer due to short cycling and condensation. Go with the 60,000 and be done, trust me that's what I do. As far as brand sure you can go find anything at any store possible but when it comes to parts, and it will eventually, you need to stick with a name brand. Reznor, Modine, Hot dawg, and ADP are all great heaters. Personally, from a service mans perspective, the Reznors are the noisier of the above mentioned, vibration noises on warmup. I personally am the bigger fan of the ADP heaters. As far as sidewall venting goes-check your install vent requirements, many want you 2' past your overhang of the roof, so if you don't mind a 4' pipe hanging straight out the side than ok, up is more aesthetically pleasing to most, slope it downward also if you sidewall it. There's my 2 cents
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And if your gonna keep the space heated all winter keep it warmer than 45 or you'll rust it out also, you'll need a garage stay, many house stats low end is 55
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My 60,000 does my 1200 square ft with 11' sidewalls with no issues