The document that LOONRAT has provided a link is an excellent resource. Those countries that on the face of it demand a transponder, such as Hungary, may not be absolutely rigid on this. Its worth a phone call for an exemption. ADS-B may be acceptable in place of a transponder, you can only ask.
NATS are very keen on ADS-B in and out and have sponsored and supported a lot of research into the low powered and economical benefits for light aircraft that are now available. NATS are responsible for achieving the rapid approvals achieved for these units in the UK. The advantages to ATC are enormous as well as benefiting the pilots spacial awareness. Transponders however on their own are a one way benefit.
The suggestion that you find an old unwanted cheap transponder and lash it together uncalibrated to me is unacceptable.