Buying off the shelf is more convenient if you have less than quite a bit of understanding in electronics.
I designed my own DCC system from scratch, it’s cheaper than buying off the shelf so long as you don’t have to pay for the education in order to know how to do it.
Somewhere out there on the www is a guide I made to making your own decoders, point motors, control panels, signalling and scratch made system.
I say it’s out there somewhere, its a free site and I don’t have the login details anymore to take it down. I get hundreds of emails a year asking for help and further info and I just don’t reply, I don’t have the time to support most of the people that look at it.
The beauty of those from non electronic engineering/computer science background people buying from companies or shops is that you have someone whom is paid to (hopefully) help you when it goes awry. Although many companies these days outsource tech support to ‘forums’ where users can fulfil an obligation to their customer for free. I no longer participate in any DCC forums because I realised that I no longer wanted to provide free advice and support for companies who provided me with nothing in return.
Free intellectual property comes with tech support that matches the price you paid for the information. If you use my designs (or anyone else’s - mine is not necessarily best, easiest or cheapest) and you can make it work, great! However, if you cannot workout the calibration of the point motors be prepared to break points, bend moving signals, burn motors, have the wrong lights illuminated on panels and be really disappointed when you email and get no response - I barely even look at the email on the site.
The one piece of advice I will give if you really want to build your own point decoders is contrary to advice above, avoid any design that mentions PICAXE or Arduino, both are certain to be woeful. Go with a design that uses PIC (or ATMEL which is also okay) not the ‘kiddie’ variants of these which will have all sorts of time handling issues giving you intermittent cases of working or not working and you’ll never know why.