Ok friend .. To start this discution ..you need to tell what type og materials do you want to weld .. Steel .. aluminum? what thickness?
all that will directly define the machine .. I restore old british cars so i replace panels and try to repaire them to lool like factory ..my welder is a portable 6500 AMPS welder .. i very heavy but still remains manual . I decided to buy it .Because to design it would have been too expensive .. it did cost me $450 ..last time i saw a chinese copy cat going for $175 Well now you can't beat that .. I did a lot of research to try to make a chip one out of a Stick WELDER .and i also discovered several INTERNET DESIGNS .. Bust firts tell me what are your needs ..My welder is very limited .TO WELD thicker things i would need more current .
One thing thst is very IMPORTANt ate THE TONGS .. .I mean the copper contacts .. thsi have to be o the purest material and THE PRESSURE between the two TONGS or electrodes is very LIGHT .. Don't clamp things by using the tongs .. the trick is to have high resistivity at the ELECTRODE MATERIAL contact .. Some manufaturers of PROFESIONAL equipment will make tables avalaible for each type of material !
One more thing Depending of the POWER requirements is that you need some control OR NOT .. most of them don't have any control .. IS BASICALLY just a TRASNFORMER with a switch .. but the ONLY CONTROL may be a TIMER to DOSE the AMOUNT of CURRENT .. SO NO FANCY ELECTRONICS HERE .. just a little TIMER !