Material needed to make a buggy
The first and most important thing is to acquire an engine. You can go to the scrapyard and get expert advice, but the most recommended thing is that it has between 500cc of one cylinder and a maximum of 4 cylinders and 1000cc.
You will also need metal tubes with which you can form both the external and internal structure.
And finally, you will have to have the rest of the parts: steering wheel, wheels, shock absorbers, gearbox, brakes, seats, suspensions... The most economical thing is that you go to a scrapyard, as they will have all the parts at a more affordable price and, in addition, they will advise you well.

How to choose the engine for the buggy
Fundamentally you have to take two factors into account: the type of driving you are going to do and the money you are willing to spend. If you take a water-cooled engine you can place it on the back of the buggy, where the radiator will also go, thus favoring the passage of air currents. If you decide on a motorcycle engine, you can use it from 500 cc with one cylinder and four strokes to 1,000 cc engines with four cylinders.
What terrain are you going to drive on? Depending on the terrain on which you will drive the buggy, one type of engine or another will suit you. For example, if what you want is to go at high speed on firm, rocky or muddy terrain, a multi-cylinder engine is best. But if what you prefer is sandy or swampy terrain, it is best to go with low-revving power engines, such as a twin-cylinder or a single-cylinder dirt bike engine.

How to make the chassis
To carry out its preparation, the first thing you have to do is clear the workplace. Stand on a flat concrete floor and start cutting all the pieces to start with the lower chassis frame.
To begin you must draw a plan with the exact measurements to scale and follow it to the perfection. It will help you visualize your process at all times.
First start by making the internal structure, the bottom part where the wheels will go and where you will include the seats. From there you can lift the rest of the buggy.
Before welding, keep in mind that all the pieces have to fit together, always check the angles and do it for both the upper and lower chassis. It is important that you connect both parts of the chassis with the anti-tips and the vertical supports.

Here you have some basic notions of how you can make your own buggy, but if you have any questions it is best to contact specialists who can help you.
You can see some tutorials, we recommend this one: