I do not have any 220 Outlets in my house. Got a welder with a 115v or 230v input. I'm set up for 110 outlets but would like to switch to the 230 input. Considering purchasing on of those transformers that plugs into a 110 and converts to 220. Got a 42 Amp draw on the welder and a max 5000W on the Converter. Is this a good Idea?

Not unless you have a 50 amp 110 outlet, which you don't. If you have to have a special outlet wired in, make it a common readily available 30 amp 240 with #10 wire.

What voltage will the 42 amps be at? The 5 KW transformer, can only supply 20 amps at 240 volts, and the 120 volt input will draw 41 amps.

At labman states, since you only have 120 volt outlets in the house, the transformer does not have a circuit or outlet to use.

Will be cheaper to not purchase the transformer and install the special outlet and circuit for it, just install a 240 volt outlet rated for the welder amp draw and be done with it.

Will be safer also.

Before doing this, need to be sure you have 240 volts available at you panel or service.