i would not go the used part route unless you want to go from one problem to the next , as the stripes are more common then rain in a british summer
so a used logicboard is likely to devellop the same fault or others very soon , as a used board has the same age , and the faults come with age , a reason why those early intel iMac's can now be had for less then the price of a equal Mac mini (same processor) here in the UK as more and more iMac's fail (display /logicboard )
so sell as quick as you physiacally can for as much as possible
getting a Mac is like the property ladder , use it for 1-2 years then sell it with profit and get a newer better one ,keep it 1-2 years and sell for profit ,get a new one, keep it for 1-2 years andsell for profit , get a bigger and newer......... and you never again spend more then £200 tops on a new Mac
if you dont follow that strategy it will get expensive to keep up with repairs after apple care ended