There's nothing inherent to say that using a used part won't work. Most likely your mechanic was advising against it as it can prove to be a false economy because of the amount of labor involved in changing them over and there's really no way of knowing how long a used one will last, if it fails a week later you've wasted the money, if it lasts a year (your target) then the gamble has paid off.
Assuming you plan on keeping the car for a few years then it's almost always going to be cheaper in the long run to replace with a new part - but sometimes you have to go with what you can afford at the time.
In your case however, you propose doing the swap yourself so you aren't paying the labor cost and the price of the part becomes the major factor.
Where I would say it's worth getting new parts is the gaskets and bolts - it'll cost you peanuts and many gaskets won't re-seal properly when you reused them (as they seal initially by getting compressed during the initial fitting) and you'll be heartily ticked off if you put it all back together and find that it's now blowing from a re-used gasket! As for the bolts - they will take quite a bit of stress when you remove them and given their age you don't want to risk having one shear off when you try and torque them back up.