I think it is a greate idea to build your own if you can, as you will understand your bike and be able to fix it later if there are any issues. And you will save a LOT of money and get a higher performing bike, in general.

This is possibly the finest ebike parts site - www.ebikes.ca. I recommend that you read the information on the web site, for general information about hub motor conversions, and try out the simulator for different motors and battery types, in relation to required power levels and top speed and torque requirements. They also supply a wide variety of parts, including the famous "CycleAnalyst" which is an excellent device for monitoring the state of your ebikes power consumption and energy consumption. Although they are "in the colonies", you may find that they will ship to the U.K. The support you will get is excellent from them.

Major questions you need to answer...

Hub motor or "through gears" chain drive (hubbies are easiest for beginners). Chain drives are complicated and often difficult to realize with custom freewheels and chain tension issues etc. And they may be less reliable in wet conditions and much louder. I'd go hub motor as a start.

Front or rear conversion? I think front is simpler for a beginner and gives better weight distribution on the bike, without having to build a "mid frame" battery carrying solution.

Will my hub motor fit into my dropouts? (ebikes.ca has engineering diagrams for their motors_). Standard is 100mm on front and 135mm on back for inter-dropout spacing width.

Pedalfirst(start immediate) or sensorless (pedal first) controller?

Throttle or PAS? a.ka. pedelec. PAS means pedalling is not optional.

What voltage should I run at? (Governs top speed).

Type of hub motor (direct drive or geared?). Geared are lighter, smaller, and more efficient, but often can't run sensorless, so may have two additional points of failure - the Hall sensors found in the motor, and the gears breaking down under conditions of excessive continuous power.

Where to put the battery. On rear rack or in the triangle. Rear conversions are "back heavy" if you put the motor and the battery on the back. Not good handling. I like front conversions with battery on back rack for most simple conversion for beginners.

Torque arms - can your fork accept them. There are different types. Should have for peace of mind.

Thankfully, cheap bikes are often hi-ten steel with steel forks so they are often better for converting than high end bikes.

Braking power? Disk brakes are nice with the additional weight of an ebike that you need to stop, and the increased brake wear.

Must have regen? Or not. Not that important in my mind. I don't miss having regen on my various conversions I've done. Regen entails some complexity to interface with battery in some cases, such as rewiring with diodes. Not worth it for beginners.

Is your fork suspended or alloy? Not so good, if front conversion. Prefer steel or cromolly. Ok if back, but need torque solution.

Wiring and connectors...you may wish to familiarize yourself with "How to solder an Anderson connector" (Google it). These are great DC wire connectors. They are a bit mysterious at first but are easy to use. Soldering them is easier than crimping them, without a special crimp tool. I just torch them and let the solder flow into the end of the connector. Very secure.

WWW.BMSBATTERY.COM and www.PINGBATTERY.COM are also key sites.