As with all modifications, the interior is no different, it's a matter of taste - what works for some etc...
One of the frequent questions is regarding the seats so the emphasis here is firstly on seats! The interior is a place where you spend the most time when with your MG, so it makes sense to have it as comfortable or as efficient a place to be as possible. In some cases, interior modifications are done for performance reasons - more supportive seats, reducing the mass, making it safer etc. In other cases it is to make it more comfortable and more upmarket.
In fact, the interior will be a very functional place with the bare minimum of aesthetics, but maximum ergonomics. It will not have carpeting, sound-proofing or centre consoles etc, and the back seat will probably be removed on the BGT.
On the comfort scale, the usual upgrades to form and function include:
more comfortable leather seats
more luxurious carpeting and trim panels
a Moto-Lita steering wheel
walnut veneer dash and consoles
magnolia faced dials
electric windows and central locking
a decent sound system (if it can be heard)
lots of chrome bits, or in the modern vernacular, "Bling'd" in the truest Magpie manner.
a decent hood on the Roadster or sunroof on the BGT.
A fair proportion of a rebuild budget can be swallowed up in this area, as seats and carpeting alone can reach £900 before the cost of walnut etc. A fully specified interior could easily run to £2000.

If you are going racing or rallying, then the list will probably include, in line with the regulations:
4 point harnesses
roll cage
bucket seats
sports steering wheel
alloy pedals
passenger foot brace
fire extinguisher
easily visible dials
easily "reachable" switches
Whether you choose to go the one route or the other there is no right and wrong - it's your car. Of course, if you are keen on originality, then you won't find much of use on these pages.
What is you want to achieve? A sympathetic improvement in keeping with the classic character or an all out modern apporach? Or maybe a racing theme to go with the period race look?