I would echo everyone above that the first thing you want to do is give it a good service and get it structurally and machanically sound first befroe doing naything to the body work.
Having siad that My series 3 is a very late model (1982) and I have gone through little changes to make it look cosmetically older than it is. With yours where is the dash clocks, are they in the center (correct) or behind the steering wheel (series 3) this is the largest and most difficult to change visual difference between the two models.
If you wanted to go back to lights in the grill you would need a new radiator panel, grill and wings, however you can convert a series 3 front panel to a take a series 2a Maltese cross grill. I have done it with my series 3. In order to do the conversion you need the maltes cross grill, the pressed land rover badge, and the hook type brackets mentioned by
@callisr. I got my grill and badge from ebay, there are "plenty" available but they are not cheap unfortunately. The hook brackets can be bought new fr not very much money for "the welding wood" on facebook. they are great and very easy to deal with.
You need to mark and drill a new pair of holes for the badge mounting screws, I filled them square and used the plastic inserts the same as the series 3 grill used. the mounting tabs for the hook brackets are already there at the bottom of the panel and just need drilling out to rivet the brackets on.
As has mentioned above you also have later series 3 door hinges and mirrors. The proper series 2a hinges (especially with the mirror mount top hinge) are difficult to find and expensive when you do. What I did with mine is just swap out the nwer style wing mirror for the older two price one more similar to the correct version.
picture of mine which you can see superficially looks more like a late 2a now than a 3:
