Yup (to both). There are loads of dead people who have proved their excellence at low level aerobatics. As for the lowest level, it all depends on who you are, what you are doing and where you are doing it. The basic law is the 500' rule (from person's, vehicles, vessels and structures) and also no aerobatics over congested areas. GASIL suggests that aerobatics should be completed no lower than 3,000' AGL. Also, one shouldn't recklessly endanger and aircraft or those on the ground. To legally perform low level displays, you need a Display Authorisation and these are issued after a test of competence by a CAA examiner.
PM