You can run both. Start running it as a bitcoin node. You can choose either core or knots. You can add lightning later if you want. I was lucky and got a used m710 under 50€ but had to flash the bios. I guess a fully operational one would cost you about 100€. I would go at least woth an i5. Ram can be upgraded later. 2TB ssd about 200€ new. So you should be able to build it for about 300€ total. Maybe less if you find another mini pc. HP has also a good series called prodesk or elitedesk