As a Scan Doubler / Flicker Fixer, I use the cheap and cheerful GBS-8220. These can be bought new from eBay for around 20 Euros including shipping and with extremely little modification work surprisingly great with any Amiga.
Works great with any LCD monitor i tried and does an excellent job flicker-fixing.
I have an Indivision AGA on my A1200 (worth approx 150 - 170 euros second hand) and the GBS-8220 on my A2000 and I can honestly say that there is not much difference is quality between them.
