Buying a Royal Enfield is slightly different than buying any ordinary bike. Though there are dealerships in several countries, the bike is manufactured only at one place. It is in Chennai, India. One should be already aware that it is a classic bike, still keeping its retro looks. Though it is not using the bleeding edge technology like other bikes, several upgrades had been done lately.
In a witty tone, once my friend told that having the Royal Enfield is like having the wife. You take care of your bike, and it will take care of you.
I have bought a 4 yr old Royal Electra in India for half the price of a new bike, and it ran good for my daily commute, and long rides. The only money I spent during four years I had my Royal Enfield was regular oil change, and replacement of a clutch cable.
First of all you need to decide if you are going to buy a very old junk Royal Enfield, and give it to a mechanic to rebuild it, changing literally all parts and making it shine and run like new one. I know of several specialized Royal Enfield mechanics in India who can do this. Without exaggerating, I can tell you that the bike will look, and run like new. However the price of buying the old bike, the price for all new parts, paint work, and the labor (which is cheap in India) will add on to a good amount, which will be equivalent to buying a 6 year old decent Royal Enfield.
The other option is to buy a not-that-old decent bike. This time ensure that you test the vehicle thoroughly. Do not buy an abused bike to save a bit of money. If you do this you will not enjoy your rides, and will eventually spent lot of money fixing it. Do ask the user how he rides the bike. If you get a feeling that the person is a fast rider and loves cursing it above 70 km per hour, please do not go for that bike. From my personal experience, having owned two Royal Enfield bikes, they are not meant for fast rides, especially the models prior to 2005. They run good if you maintain a 55 per hour ride. If you ride these bikes at higher RPMs you will mess-up the engine.
The best way to understand the condition is by hearing its sound. Take the bike to a quiet place, switch the gear in neutral position and start the engine. Pay close attention to its sound from the engine. Make sure the sound is tight, without rattling noises. Ideally a person who maintains the bike well, when not using the accelerator or as we say when in idle, will keep the rpm as low as possible, just for the engine to run without stopping. Check if the sound from the engine is smooth, following a stable rhythm. If there are lots of noises (not the good packed sound) from the engine you may want to avoid that bike. Now increase the acceleration bringing the RPM to 3/8 of the max and notice the sound. If it still sounds good, then that is a good sign.
Examine the sprocket attached to the rare wheel. If you see that the teeth are narrow and worn out, or if there are any missing in between, that will need to be repaired. Riding with this condition will spoil the chain, and engine.
Examine the body of the bike. Is the paint work in a good state? Do you see any damaged parts, or any signs that the bike was involved in any serious accident? How about when you ride the bike at 40 to 55 kms per hour? Do you feel any vibration, as if the wheels are not balanced or aligned properly? You need to check all these when buying the bike.
Well, best of luck !!!