a set of mtb spd’s (the usual flats on one side spd on other)
That’s not really ‘the usual’.. that is either touring pedals, or those god awful semi cage, semi spud things. They will make it harder for you to make the transition to spds… get some decent double sided ones or you’ll be forever trying to flip the pedal over before you clip in.
stupid question part…what do i need to allow for, is it just buy any old pair of spd shoes and away i go?
Pretty much, yeah – make sure you’ve got some cleats – they normally come with the pedals, but if you’re getting them 2nd hand, you might need to buy some separately.
im aiming to use the pedals on my fixie if that is an issue?
No issues beyond the usual questions to your mental health related to riding fixed 😉
Dave