Starting cycling on a budget

Hi guys,

Dont rip me apart as most of you must be hardcore cyclists but please suggest according to my needs. I'm 100kg, 5ft 10 inches and thinking about cycling to lose weight and excercise my knees. I went to local store and liked a 26 inch Hero cycle with single speed, both disc brakes and front suspension. But now have a few questions in my mind. 1. Is suspension required? I weigh 100kg 2. Rim brakes or disc brakes? 3. MTB with fat tyres or road oriented? Shopkeeper suggested MTB, claiming it will be better for my purpose. 4. I dont think gear will be possible in my budget but will single speed suit my purpose? 5. Suggest size for me. Im 5'10". 26 inch wheel?

Please guide me and if possible suggest a model.

Budget is 7.5k max