If you're interested in Newton programming, here are some links to pages that may be worth reading:

http://www.cocoadev.com/index.pl?NewtonOS

http://www.panix.com/~clay/newton/query.cgi?programming+index

http://www.cat2.com/newton/Newton-FAQ/newton-faq-development.html#VIIA1b

http://myapplenewton.blogspot.com/2009/12/learning-newtonscript-foreword.html

http://web.archive.org/web/20041010175535/www.cc.gatech.edu/~schoedl/projects/NewtonScript/

http://newtonpoetry.com/2008/04/09/the-origins-of-newtonscript/

http://www.unna.org/unna/development/documentation/'>http://www.unna.org/unna/development/documentation/

It would be better to develop on a real or emulated mac probably, but since I assume you don't have one of those you may have to go elsewhere. If you do have one that's recent enough to run mac os 8 or mac os 9 great, otherwise you'll have to use window or something else. You can get the development tools (they're pretty old now) over here: http://www.unna.org/unna/development/NewtonDev/ You can get other software over there on http://www.unna.org/ as well.

That said, you might prefer the ZX Spectrum +2 that you mention. It's probably less troublesome.