Looking to buy an analogue synth (not necessarily a modern one, so vintage is on the cards). Ideally it would have a keyboard and MIDI output and input so I could use it to send MIDI to Live to record in sequences and then Live could send MIDI back to it to make it play audio. I mainly want something with plenty of parameters to tweak and muck around with and am looking for pad/lead sounds. Any advice?
Probably of prime importance is the question: What is your budget?
I am certainly not an expert like Marc Doty (look up automatic gainsay on youtube) who seems to have road-tested every synth in existence, but can try and help, I'm sure many others here will have useful advice to contribute too. Sometimes 'vintage ones' cost way more than modern alternatives. Don't necessarily discount a rack-mount or desktop option as a good controller keyboard can serve you well with multiple instruments both hardware & virtual, velocity sensitivity is a must and aftertouch is desirable if you are looking at that option however. (I've found the keyboards that come with Arturia's analogue experience range to be reasonably good)
Due to the complexity and cost of components most analogue synths are monophonic, once you get to the polyphonic area the cost really starts going up, (although the newer options out there like Elektron's Analog4 and Dave Smith's Mophox4 look interesting!) - but don't discount the monophonics for pad sounds as you can spread the tuning of the oscillators on units with more than one oscillator to get nice pad sounds and depending on the options of any given unit the filtering options can spread/pan the sound across a stereo field too.
Different makes/brands of analogue synths and the way their circuits are built, the type of filters used etc will all give each a unique kind of sound If you can, do some homework and do a hands-on with a few potentials on your list to hear what they sound like (keeping in mind the amp/speakers they are going through in any given store can affect the sound)