My advice would be 6061 T6 aluminium. Very strong and good corrosion resistance. Easy to machine and anodizes well. Even left unfinished it looks good for a long time.

7075 T6 would also work extremely well as it's harder and stronger than the 6061 T6, but the natural oxidization of the surface doesn't look too pretty so probably needs to be anodized to stay looking good.

5083 H112 is another good aluminium and the cheapest of these three, with 7075 T6 being the most expensive. I use 5083 for knocking up sample step kits to check for clearances and position and what have you. Then knock out the real thing in 6061 T6.

Be insistent on the temper of the material you want, as it makes a huge difference. For example, if you're fobbed off with 6061 T0 then although the grade of aluminum is correct, the temper (T0) is softer than a very soft thing so no good for suspension link plates.