I just bought Freespace 2 from the sale, and before I start, are there any "must haves" in terms of graphical or any other mods for the basic campaign?

Download FSO with all the bonus mission.
Go to sleep ... the download will take a lot of time.

To expand on what Pulzarokkit said, you can download the FSO engine from here:
http://scp.indiegames.us/bnr_installer.php

It's pretty straightforward, but here is a getting started page to familiarize yourself with the FSO installation process:
http://www.hard-light.net/wiki/index.php/Getting_started

The Freespace Open Engine is really a vast improvement over the vanilla game for both FS1 and FS2 - beautiful HD graphics, compatibility fixes, etc ... I would highly recommend getting it.

They are right, you should totally get the Freespace Open (FSO) it gives you the opportunity of using many mods and it´s very user-friendly. There are many mods and user-made campaings (Some of them of excellent quality). If you´re using windows this is the best guide you can follow to install it manually: http://www.hard-light.net/forums/index.php?topic=78710.0 (Other guides or using the installer may make you miss some steps such as the OpenAL one).

Even though there are many mods to choose from I higly recommend you two of them: If you haven´t played the first FS you should get the "fsport" which is a conversion of the first campaing to the second game and also the "mediavps" mod which gives the game some pretty awesome graphics. Really, really awesome graphics.

Anyway in the hard-light website you can find all the information you need.

I'm with the above. Get ALL of the new mediavp enhancements. Sound, visuals, everything. It's around 800 Mb total IIRC but worth it in sooo many ways.

If you really want an appreciation for SCP and the FreeSpace Upgrade Project, play vanilla stock freespace first. Then play through SCP. It's like playing a brand new game.

As for mods, I recommend playing the following:

Playing In order:
-The FreeSpace Port -basically FreeSpace 1 ported to FreeSpace 2.
-Silent Threat Reborn -covers the Hades Rebellion. This is a re-imagining of the Silent Threat expansion for the original FS. ST had a good story, but overall crappy mission design. ST:R is fully voice acted, excellent missions, and very enjoyable. It's also quite difficult at times.
-FreeSpace 2 -the main FS:2 campaign. Play all with the MediaVPs. It's well worth it.

In no particular order:
Derelict -an older campaign for FS2, updated to work with SCP and voice-acted. A little dated in design by "modern" FS standards, but awesome story and fun gameplay.

Blue Planet Age of Aquarius -IMO, to date, one of the best FreeSpace 2 campaigns. Full voice acting and a much more "personal" story.

Blue Planet: War in Heaven -Not voice acted, but totally awesome nonetheless.

Total Conversions:

The Babylon Project -Babylon 5 on the SCP engine. Amazing effort put forth into this TC. It's seriously better than a lot of games you pay $60 for.

Wing Command Sega -Not my cup of tea personally (I was never a huge fan of the WC games) but it's very well done and very polished.

Thanks for all the suggestions! I actually own FreeSpace 1 + Silent Threat as a boxed copy, so have played those through a few times. I installed FSO, but haven't downloaded the improved graphics yet as there seems to be a lot of different files and updates to download (the size of the files is a non-issue).

You can find the last set of the mediavps here http://www.hard-light.net/forums/index.php?topic=70736.0

You just have to download the files below "[DOWNLOAD OPTION 2] - The individual component VPs:" and get all of them into a folder called mediavps_3612 which has to be in your main freespace 2 folder. Then when you open the launcher select that folder as the mod you want to use and go to the "features" part of the launcher and select "turn everything on". If then you max the settings within the game you´ll have a REALLY awesome-looking space sim.

Is very important to download the mediavps because most of the other mods make use of it too.

I must note that you don't need to own Freespace for the wing commander saga, on more notes, it may sound weird but
Don't buy the first game, not because it's bad, it's really good, but the guys over on the hard-light forum have released a redux of both the first game and it's expansion, which to my knowledge you don't need the first game for, experience it that way, it's still amazing to play and all that.