This is an FPGA I'm using for a non-Spectrum related project, actually BBC Micro related.
However, it would make a good basis for implementing (at least a 48k) Spectrum:
- Has a fully open source toolchain (yosys, nextpnr, Icestorm tools) which are considerably nicer and lighter weight than the typical proprietary vendor IDE
- Has 128k of static RAM built into the chip, so you can certainly implement a 48k model with no external RAM required. Also has 15k of dual ported block ram in addition.
- Has a built in PLL for clock synthesis
- is cheap
- is available as a non-BGA package
There is a development board for this chip (the Upduino 3.1) for around 30 euro. The upduino board includes all you need including the USB FTDI programmer and a decent amount of serial flash for storing code, and you've enough GPIOs to at least be able to do RGB and support the standard Spectrum keyboard membrane.