So I think I have read that boiling in aluminum is bad, you should always use stainless steel. But will this really put off flavors in my beer? I want to buy an outdoor boiling system but the ss is too expensive.
No it's fine.
Just don't use caustic cleaners or scrub it bright.
Just not quite as durable as stainless.
An old keg will normally work out cheaper than a stockpot though.
I brewed many great extract brews in alloy kettles. don't stress it!
There are some advantages of aluminum over stainless steel, too. It is lighter for one, and it has better heat dissipation than an all SS pot (less chance of scorching).