In the midwest you can find a mountain bike or 10 speed for $50 at a charity resale shop or a flea market. For a large sized frame to fit a 6'er, you may have to go to craigslist and spend $150-200. don't buy carbon or titanium, light weight and motor wheels are incompatible. How handy are you? Usually with a bargain bike there will be issues with the brake cables and maybe the shifter, perhaps requiring replacement with the $3 cables from the discount store. If you find one with bolt on pedal arms, that is the kind the wheel kits have a pedal sensor for. Usually tires will be flat, the tube has to be replaced, if cracked the tire too.
You can get a powered wheel for $150-200 including motor, wheel, controller, pedal sensor, brake shutoff controls, display, not the tire or battery. Adapter and 7 speed sprocket cluster for rear installation is extra. Don't put 7 speed cluster in a really old (pre 1995) bike which was 5 speed originally, the old chain is too wide for 3/32" sprocket teeth. You can get a battery for $150 up, make sure the voltage is compatible with the motor. If your area has steep hills, 10-15% I don't recommend mid-drive because they eliminate the lower tooth count sprocket of a mountain bike. I bought a ebikling geared wheel, which was complete with a internal one-way clutch, as I have 15% hills I have to conquer plus I intend to pedal unpowered a lot on long (80mile) trips. Make sure motor wheel will fit inside your bikes forks spread. you may need a huge pliers or allthread/coupler nut combination to spread the forks. You may need a file or grinder to open up the fork slot a little - I did. You'll need a crimp tool to put connectors on wires. The ebikling used barrel connectors .157" from the auto supply (dorman) to connect the battery. I used insulated flag terminals for the charge wires of the battery, male for minus, female for plus. Don't uses ****ese crimp connectors at 30 amps, use taiwanese (dorman) or US (3m, panduit, ideal, T&B). The ****ese ones melted on me on another project. Pull test your crimps before trying to use them - amateurs don't squeeze hard enough often. Longer tools give more force, I use an ideal or klein. Some bolt on pedal arms require a puller to get them off after using a socket to remove the nut. Remember one side is left hand thread.
A torque arm is useful for the shaft not spinning in the fork, I made one out of a piece of scrap steel (bed frame leg) making the custom shaft sized hole out of two holes and a grinder (use safety glasses). A file will work ifyou have more time than money. Bolt the torque arm to the accessory holes of the fork, 10-32 screws on my mountain bike. 5 mm screws on my 10 speed. Get odd screws at home store or auto supply. I double nutted the motor shaft, took 12 mm x 1.5 nuts which I had to go to a motorcycle shop to buy. I double nut on all bike drive wheels; I'm strong enough on the pedals to pull the shaft forwards in the frame and bind up the tire against the frame when I get on it up a hill.
You may find at higher speeds disk brakes are advantagious. I bought them but couldnt fit them in my fork: since I'm riding way out in the country deceleration speed is not really important. In a city you may need disk brakes more, especially in the rain. I intend to stay below 15 mph, 12 average, to extend my range between charges.
Some batteries come with a charger, some don't. The battery from btrbattery did, but a 15 ah 48 v was $435, more range and weight than you probably need. I had to make a mount for the square battery out of aluminum angle. Bagged hanger batteries or ones designed for the water bottle holder are easier to mount, cheaper, but easier to steal and hard to secure IMHO. Once I glue the screws on my mount, thieves will have to use a saw to get it off.
Don't forget the 6' steel cable (coiled) from master and a serious lock (the round ones take two cuts) from the home store. About $30. On a budget you don't want to donate your project to the neighborly thief. 6' cable to put through both the frame and the powered wheel.
Have fun saving money.
