what's your budget?
the starter machine is a recent mac mini - best you can afford, ram it up to max
add on a thunderbolt or firewire external drive for your audio/songs
a 3tb tbolt drive is about 235 gbp
add a cheap large usb drive for backups
a 2tb backup usb 2/3 drive is 100 gbp
You can use a mac mini with any cheap screen and usb mouse/keys
add cheap audio interface & mic = from 100 gbp
work up in price from there to a recent imac or a faster mac mini (or mini server) - dont underestimate the newer minis.
new prices as a guide (inc vat at 20%):
a quad imac new is starting at a 1000
a quad mini new with 4gb ram is about 675
a quad mini new with 16gb ram is about 850
a dual-core mini i5 new with 16gb ram (!) is 660
bare in mind a late 2011/12 mac mini Intel Core i7 will equal an older 8 core mac pro on geekbench.
So for example, get a quad core imac, with 4gb ram, then buy & fit 8gb or 16gb crucial ram.
total cost new = 800 for a i7 with 16gb ram new - or less with a s/hand i7
save money by adding a smaller 1tb tbolt drive (160gbp) & starting with just a smaller 1tb backup usb. (65gbp)
total 1050 gbp - if you need cheaper then go for a 16gb i5, or cheaper still try an older model i5 imac or mini
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but it depends what you really want to do in terms of how complicated you want your demos to be. If you just want to get down some software drums & other backing arrangement parts with a typical 8 or so audio tracks on top, then frankly you dont need too much power & an older dual core might be enough.
also have you budgeted for or do you have already your monitor speakers, a mic, a stand, leads and headphones?
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