I'd go ahead and apply (since being rejected won't affect any future applications), but in the meanwhile, build a prototype and see about getting a quality cofo

I'd go ahead and apply (since being rejected won't affect any future applications), but in the meanwhile, build a prototype and see about getting a quality cofounder in the meanwhile. If you don't get in, then apply next round with the cofounder... but perhaps you'll get in this round as a solo founder and you'll find one during YC.