Overall, the 3 you listed are pretty similar. I would avoid the Folgertech one, mostly because the frame has very little in the way of bracing for the vertical frame. I prefer the wood frames myself, but acrylic is ok I suppose. Of the 3 you listed, the Alunar m508 looks like the best choice. It has the print cooling fan, which is a must for PLA.
One thing about all these, they say they will print all types of materials, but anything that requires over say 240 deg C extruder temp is probably not gonna work. Mine said 5 materials, but over 240, the ptfe insert in the hot end tube breaks down.
There are a ton of generic kits that can work well too. Mine is a Geeetech i3 Pro B clone, and with a couple updates works like a champ.