You mention "Leonardo" and "Mega2560 R3", which is confusing...
Some of my thoughts:
- The Arduino Leonardo does not have so much of ROM. SD + TFT software will eat up most of the ROM. Not much will be left for your application.
- TFT is slower (more data to be transfered) than monochrome displays (if this is an issue)
- When looking for a display, check the display controller.
- Check, if there is a Arduino Library which supports the display controller
- Do NOT use "controller-less" displays without internal RAM
- KS0108, ST7565, ST7920 are monochrome display controllers which are widely used and have good support for Arduino
- Typical Arduino libraries are: http://henningkarlsen.com/electronics/library.php?id=52
(Monochrome, all variants of KS0108 displays), http://code.google.com/p/u8glib/
(ST7565, ST7920 and others)
- Have a look here: http://forum.arduino.cc/index.php?topic=134689.0
(inexpensive monochrome 128x64 display)
- sparkfun.com, adafruit.com and others not only sell displays but also provide (simple) libraries and example projects