suggest you get the 1704 composition book (eg see those for Castletown - http://www.ee.surrey.ac.uk/Contrib/manx/towns/castltwn/1704_cb.htm (the Peel one is also online) - this will have details about who compounded what in 1643, often with useful comments like 1 life left, grandfather etc (I worked from the well written 1760 copy made mostly by that indefatigable John Quayle whose handwriting is great, this copy may be on mormon film, the actual book is and I believe contains additional info like dates of death etc to check out the lives left) - this together with the 1703 will then allow you to progressively work backwards - judging by Peel cottages which I have worked on your difficulties in reading really only come c.1580's though some of the early 17th c manorial rolls appear to have been written under the influence of too much jough. But working backwards allows checking of names whereas hitting a 1573 straight off would be very difficult - however this needs either a lot of 30p copies at Manx Museum (from microfilms) and a lot of time.

Vast majority of deeds in Manx Museum are post 1702 - catch is that you need a name either of grantee or grantor to find but again they often have useful info re relatives

the song is I think
http://www.ee.surrey.ac.uk/Contrib/manx/fulltext/mb1896/p186.htm
and is one of the more famous ballads

Any transcribed wills would be most gratefully received