If you're OK with non-Mitsubishi parts try www.rockauto.com - pretty good prices & great service. I think you're safe with using aftermarket belts & hoses, plugs & wires, but I'd stick with OEM for everything else.
Ditto what Justice said above about using Iridium plugs. I beleive that Denso & NGK both have applications for the 3.5-search the web & ebay stores for the cheapest price. I'm also a firm beleiver in Magnecor wires due to many years of problem-free performance from the sets on my SR & my 240sx. That said, they aren't in everyone's budget, especially when you're doing as many other things at the same time as you're planning to do. Whetever wires you get, definately install them before you install the motor, as they really suck to get into the retainers behind the motor when it's already in the vehicle.