Hi
Although the Simplex 20hp tractor was the most common tractor used by the WDLR it is the most difficult to build. The Nigel Lawton kit contains the chassis and the body and it is small and not one for novices. The chassis is particularly difficult to get running although Nigel does offer a 'get you running' service for a fee.
The Simplex 40hp tractor came in three versions 'Open', 'Protected' and 'Armoured' and there is still a white metal kit available for it by Mosskito narrow gauge although currently out of stock. Also there is a bespoke chassis kit available for the tractor by the same supplier. The chassis requires soldering but I have assembled the body with Araldite. The kit is for the 'Protected' and 'Armoured' versions, either can be built from the kit but the 'Open' version needs careful cutting of the side panels and fabricating of the engine cover and drivers seat, radiator screen and the motor. You will need also to acquire one of the small roco diesels for a chassis (perhaps too complicated for a first build, stick to the 'Protected' or 'Armoured'). Link below
https://shop.narrowplanet.co.uk/collections/mosskito-narrow-gauge
The other option, that is, if you like 3D printed stuff is Simon Dawson's kits from Shapeways both the Simplex 20hp and 40hp. Don't forget to click on the 1/76 triangle if you are doing 009. Here is the link
http://www.rue-d-etropal.com/3D-printing/3d_printed_trains1.htm
Of course aquiring a Simplex is the first step, you will also need rolling stock. Simon's site also has a good selection but you will need to source wheel sets and couplings.
Here is the link
http://www.rue-d-etropal.com/3D-printing/3d_printed_trains2.htm
That said most of my rolling stock came from Dundas Link https://dundasmodels.co.uk/webstore/index.php
Regards
James