Painfully learned suggestions:
1- NEVER BUY A MOTORCYCLE WITHOUT A COMPRESSION TEST ON ALL CYLINDERS !!!! Below 7 -8 bars, run away !
2- On a tight budget, remember that price and maintenance cost is in DIRECT proportion with the power and number of cylinders. For instance, twin cylinders are much cheaper to fix than four cylinders engines, which costs twice as much in parts and labour to keep in good shape. There is more exhaust pipes, valves, carburator, name it !
3- The minimum size for a beginner and LIGHT passenger is 250cc. It is extremely dangerous to overload a motorcycle. Some 50cc scooters can accomodate a passenger but their speed is usually limited at 50 km/h too. If you want to ride faster, get at least a 250.
4- Double the speed, you need FOUR TIMES the distance to stop ... with a passenger it doubles again, so, get FRONT DISK BRAKES !
5- If a motorcycle is older than 15 years, remember that rubber parts and gaskets will wear off with TIME, sun and heat alone. Metal and electrical systems will corrode in damp or poor storage condition. Low KM bike does not mean good deal. Invisible internal corrosion or poor maintenance makes engine and electrical systems not economical to repair. It is not advisable to buy a motorcycle over 20 years old (parts are harder to find and prices are higher when rare )
6- Don't buy rare breeds, stay with the most popular models, so you can find recycled parts on Ebay or even locally at a fraction of new parts.
You say you prefer japanese... Up here in Canada, Honda is the least expensive brand for parts, Yamaha is close. Suzuki and Kawasaki are the least expensive to buy, but their parts (especially Kawasaki) are often outragiously expensive with little generic and third party available.
Listen to the compression test, not just to the look and sound of the engine !
As for handlebar heaters, get good gloves instead of risking to fry your electrical system with high priced gadgets.
Good luck !