There are a few minimum requirements for the instrument a student uses for lessons. Students can use either a keyboard/digital piano or an acoustic piano; find the requirements for each below

Requirements for Keyboards and Digital Pianos

There are a few minimum requirements for the instrument a student uses for lessons. Students can use either a keyboard/digital piano or an acoustic piano, but here is what it needs to have.

For keyboards:

• full set of 88 full-sized keys
• an attachable pedal that looks like an acoustic piano pedal (see example)

Donner Sustain Pedal for Keyboard, Piano Sustain Pedal Universal Damper Foot Pedal for Digital Piano MIDI Keyboard Electronic Organ Synthesizer, DK-1

• touch-sensitive keys (ie. can play loud or soft depending on how hard they are pressed)
• a keyboard/piano stand that allows the student space for their feet and legs (see pictures for examples)
  • NOT the typical "x" type stand that keyboard usually come with as the student cannot sit comfortably with this

Plixio Piano Keyboard Stand - Z Style Adjustable and Portable Heavy Duty Music Stand for Kids and Adults (Fits 54 - 88 Key Electric Pianos)

Yamaha L85 Keyboard Stand, Black

• a bench for the student to sit at (not a regular kitchen or office chair) such as the examples below
  • an adjustable bench is preferable

YAMAHA PKBB1 Adjustable Padded Keyboard X-Style Bench, Black,19.5 Inches

Neewer NW-009 Adjustable Padded Piano Bench, Music Keyboard Bench, Leather Backless Stool with Solid Hard Wood Construction, Supports up to 250 Pounds / 113 Kilograms (Black)

• for children: an adjustable footstool so that their feet are not dangling at the bench

Ideal Piano Footrest for Improved Posture and Form, with Less Fatigue. Foot Rest adjusts to The Ideal Height for Each Student.

Piano Pedal Extender Footstool 3 Pedals Design for Kids and Beginners Wide Adjustable Range (Black)

Requirements for Acoustic Pianos

• in-tune and properly maintained
• functioning keys and pedals
• a bench (see above)
• for children: an adjustable footstool so that their feet are not dangling at the bench (see above)

Notes on Acoustic Pianos

• Never buy a piano without having a technician appraise it first. Many pianos are not properly maintained and such things as having gone years without a tuning, broken keys, non-functioning pedals, a crack in the soundboard, etc, can end up costing hundreds or thousands in repair and often cost more than the worth of the piano itself.
• Acoustic pianos must be tuned every six months. Not doing so can lead to greater issues that cost even more to repair. Refer to this article for more information on why a piano must be tuned at least twice a year: https://cooperpiano.com/why-does-a-piano-have-to-be-tuned-if-we-are-not-playing-it/.
• As a rule of thumb, any free piano is not going to be worth even the effort of transporting it. As stated above, many pianos are not properly maintained and not worth the cost of restoring them.
• If looking to buy a used acoustic grand piano or upright, I recommend going through a music store or reputable dealer as they will have tuned and repaired the piano so that it is in usable condition. They will also be able to move it for you and often provide financing options.
• In general, if unable to commit to regular essential instrument maintenance, I recommend buying a quality digital piano instead of acoustic as it does not require tuning, is much easier to transport, and will be much less expensive for a comparable instrument. 

Keyboard Recommendations and Links

Below I will list some example models in order of price to give an idea of suitable instruments.

Lower End

For around $200-$400 you can get an instrument that meets all the minimum requirements I have listened above that would be adequate for a beginner. Here is the keyboard that I use for group lessons in the studio which falls in this price range which retails at $230, the ​Alesis Recital – 88 Key Digital Electric Piano / Keyboard with Semi Weighted Keys, Power Supply, Built-In Speakers and 5 Premium Voices (Amazon Exclusive)

Mid-Range

For around $400-$800 you can get a quality keyboard or digital piano that will have higher quality sampled sound and likely semi- or fully weighted keys that will feel close to an acoustic piano. This type of piano would be suitable for students through the intermediate level. Here is an example of one such model which retails at $523, the ​Yamaha P71 Digital Piano (Amazon Exclusive) Deluxe Bundle with Furniture Stand and Bench.

Upper Range

Most any digital piano over $800 is going to have fully weighted keys that feel just like an acoustic piano and excellent, realistic sound. A digital piano like this would be suitable for students up to the advanced levels. Below are the two digital pianos I have in the studio which are used for lessons, recitals, and my own personal playing.

$740, Casio PX-770 BN Privia Digital Home Piano, Brown

$1,369, ​Roland F-140R Compact 88 Console Digital Piano with Bluetooth MIDI/USB and Weighted Hammer-Action Keyboard with Ivory Feel, Stereo Speakers, Key Cover, Contemporary Black Finish (F-140R-CB)

Brands I Recommend

In general, you get what you pay for when investing in an instruments. However, I would hands down recommend any Roland digital piano. They don't many - if any - budget models of digital pianos, so just any any will be of great quality. Yamaha makes many excellent keyboards and digital pianos at many price points. Casio also has some upper end digital pianos such as the Privia and Celviano line which are excellent. I would recommend these three brands above others.