I live in the desert and it gets HOT. I also hike with DD wearing my Ergo Perfomance. I chose this model specifically because the fabric seemed most breathable.
Honestly, you shoudn't be out wondering with LO if the temp is pushing much past 86... But your child is going to be pressed up against you so guess what - you're going to get a bit warm.
As for foward facing, from what I've read the lastest info says that this is not healthy for hip development. So I purposely avoided the Beco Gemini which does allow forward facing to avoid the temptation. However having theirlegs spread IS a healthy position for an infant's hips, so again, I feel good about wearing my DD in an Ergo. Also, the child sits in it rather than dangling from their crotch.
DD is now 6 months and I just had her in the Ergo today while I grocery shopped. She could look around just fine and seemed really happy in there.
Lastly, avoid any carrier that does not have a hip belt. You may know that this take the weight off your shoulders and back and places it on your hips.