How you make models is entirely up to you: you are (hopefully) doing this for your own satisfaction.
People often dither and put off taking the plunge, only to regret not doing so sooner. All I can say is, buy a few parts, and build a wagon. You don’t need many parts: wheels & bearings, axle guards (“W-irons”), castings for springs, axle boxes and buffer stocks. 4mm scale 15” buffer heads with rams, and maybe some brake gear castings and/or etchings, plus coupling hooks. Everything else can be made from whatever material you like: metal, wood, styrene, or any combination/permutation as you see fit. As you say, it won’t break the bank.
The wagon kit is useful, and you get an idea of the size/mass of the scale and can rapidly build up a few wagons for shunting (and hence, maybe a small coal-yard diorama/cameo?) and it can be modified to create more variety, but at some point scratch building or alternatively CAD/CAM (etching and 3D printing) will be required, so I don’t necessarily think these are the best starting point for a beginner, unless they whet the appetite and encourage the assembler to start extending (or applying) their skills. Whether that’s hand-work, machine work, or computer based is irrelevant. A lot of the late Stan Garlick’s work was accurate machining followed by hand finishing, and very nicely done, too.
On your own, it is only really possible to control one train at a time, so the fact that a small amount of stock requires a small layout (and vice versa) is a good place to start. With careful design, it is easy to allow for expansion of the stud, and even a second operator, e.g. a goods only tramway, or extension to model the whole branch (Yaxbury!)
I think Wicken (MRJ 10) and Lydham Heath (see the link in my signature) are still outstanding examples of simple but effective layouts.