Because the cost of a high-end modern private aircraft may be tens of millions of dollars, lending for aircraft purchases is accompanied by a security interest in the aircraft, so that the aircraft may be repossessed in event of nonpayment. It is generally very difficult for borrowers to obtain affordable private unsecured financing of an aircraft purchase, unless the borrower is deemed particularly creditworthy (e.g. an established carrier with high equity and a steady cash flow).
Though the early Lockheed Jetstar had four, most production business jets have two jet engines, mostly rear-mounted podded engine. If mounted below their low wing, it wouldn't allow sufficient engine clearance without a too long landing gear. The HondaJet is the exception with its over the wing engine pods. Dassault Falcon still builds three-engine models derived from the Falcon 50, and the very light jet market has seen several single-engine design concepts and the introduction of the Cirrus Vision SF50 in 2016.