Speedy 3520.53 help and advice please

Where did you source that picture? Is it one you are considering? That one looks to have a degraded or possibly water damaged dial so keep looking if so.

OK fair enough so:

Can I wear one as a daily? - Certainly yes, it is a very practical watch, reliable and tough, the sliding bracelet clasp is a bit of a weak spot though so be careful with that.

How much and difficult are these to service? - Easier than most chronographs, much easier than the similar looking Reduced models, The Mk40 uses a 7750 derivative for which parts are comparatively easy to get. You will have options other than Omega for movement issues, but case parts or hands might be tougher.

Where is the best place to buy one? - A question with a wide scope indeed. Ebay, here, TZ-UK, etc etc. Personally I'd avoid c24 as stuff on there is stupidly over priced.

How important are boxes and cards? - Not very, you can't wear them, but they do have value, around 7.5% each in fact, ie a watch without them should be circa 85% of the price of one with, but opinions differ.

What are they like to 'life with' ie comfy to wear etc. - The have less sharp edges then some designs so are pretty good, see q1 above!