I would not install metal roofing on a roof with such low slope. Metal roofs on very low slopes or flat roofs must be completely waterproof (unlike metal roofing on a steeply pitched roof which act more like shingles). A 2-ply, torched down, modified bitumen roof is what I would recommend, however it requires a certain degree of expertise to install it. Therefore, is not inexpensive. If my main criteria was to spend very little money, I would be inclined to use roll roofing, cemented in place. It is essentially the same type of roof described above, using less expensive materials (and consequently far less durable) and a less expensive form of adhesion. To a certain degree, you get what you pay for. The modified bitumen roof will last two or three times as long as roll roofing.