L500 or L700 should be a good start. They're pretty cheap used and not that hard to drive. The main issue with estats is that a lot of them are very difficult to drive, and amps that can power them properly tend to be very expensive. This creates a pretty big barrier to entry - but with the Lambdas you don't have to worry about it as much.
For amps, I would say a used 313, 323s, or 353x would be good, and a used 717 even better, or one of the tube amps. However with the older amps like the 313 and 717, make sure they've had the caps replaced cause those do have a limited shelf life. The Stax-branded tube amps don't have a lot of output power (unless they've had the CCD mod done and 6S4A tubes swapped in) but they do sound great at lower to medium volumes. The solid state amps sound a bit sharper and brighter but they will give you more power and volume. The best amps would be stuff like the BHSE or KGST but those have hefty price tags.
The Lambdas also equalize very well, so whatever deficiencies exist with the tuning - and they will likely be a bit bright for you - can be pretty easily fixed with EQ. The mids in particular EQ quite well, and the midrange honk they have from the 1.2khz bump goes away completely.
The fit can be an issue for some people, but you can always swap in a mk2 headband or get stuff like the Socas freedom yokes and various other mods. Generally speaking, if you get a severe bass rolloff, you're probably not getting a good fit. The bass should be relatively linear and extended but it can be tricky to get there sometimes. This isn't just a Lambda thing - the fit has to be snug with any electrostat or you're losing bass below 50hz.
If you want to upgrade from there, a better amp would open them up a lot, and will then set you up to be able to move on to a higher-end estat that needs a lot of power. Though to be honest, the L700 in the right system is competitive with a lot of TOTL non-estat setups and beats most of them in some respects.
Finally, you better have some well-recorded files in your collection because a good estat system will lay every flaw bare. You'll be dealing with a level of resolution that even most mastering labs don't have access to.
X7 amp you can change between triode / ultralinear modes. Triode gives sharper cleaner sound and less gain and ultralinear is opposite of that. Warmer sound, dirtier and more gain. 
