There are people on this forum that can tell you virtually all there is to know about a particular Mauser rifle from the font of one numeral stamped on a single metal part (almost). Likewise, there are people that have been shooting civilian service rifle (CSR) and historic service rifle (HSR) in the UK for 50 years. I am neither of those so all I can give are observations from my experience, which of course you are free to consider or discard as you see fit.
My first suggestion is to research the different CSR and HSR classes because some rifles can be used in very few matches (K98k being one of them) and some can cover most of them (Enfields). The place to start is the UK National Rifle Association CSR and HSR handbook and the 'Trafalgar' meeting handbook, available on the Bisley website. All the classes and matches are listed in there.
My second is that if you want to do formal CSR and HSR in the UK and you don't plan to have a whole collection of rifles, you're better off starting with a Lee Enfield No4 from the 1950s. Unlike K98ks, there's lots of them here, often in very good condition because they haven't seen much or any military service, the .303 is more than capable of putting a hole in some paper at any reasonable distance and it is much more forgiving and cheaper to shoot than 8x57. It's also great fun. Every other shooter on the range can help you get it zeroed nicely. You won't have to reload as much mid shoot, saving you time in timed competitions. You can get a good 1950s Enfield with a shiny bore and all matching numbers for about £700, although if you want an original scope there's plenty around but they'll cost you £5k plus. Even more important, you have a wide range of matches and classes to enter with an Enfield.
If you choose a K98k in formal HSR, you will be relegated to the 'all comers' class. That means competing against someone with a pristine M1903-A4 who will probably out shoot your score by a factor of three unless you have a similarly pristine originally scoped K98k, which will cost you a fortune and probably need to be imported from the US.
Sorry if the above sounds negative. If you have your heart set on a K98k, you can absolutely shoot and have a super blast, but you are very unlikely to be in competitive contention simply because of how the rules are written. And if you are struggling to get your shots on target, chances are no-one on the range will be able to help you out much.
Third. If we're talking informal CSR at a club, then you've generally no worry about different matches and classes. Can you compete in CSR with a bolt action service rifle? Yes. There's a good chance you won't come last. But you almost certainly won't come first or even in the top half of the table. That will be dominated by AR-style rifles using lower recoil rounds, magazines for quick reloading, and optics optimised for the specific course of fire. However, everyone will respect you greatly for taking the difficult option and you will make lots of friends.
Fourth. Full bore target shooting for your own enjoyment. Super. But you'll still need a rifle with a decent bore, which circles back to our previous posts. Personally, I would start with iron sights because you'll have a greater choice of rifle examples, but it depends on your eyesight and how much shooting experience you have. If you're a dab hand with a scope, of course crack on.
In summary. By all means get the K98k. But be clear about the competitive limitations. Take your time to find a good one with at least mostly matching numbers (including captures) and a decent bore. There's usually one or two every few months popping up in the UK, plus about three times as many that are not much more than a mess of parts and that, if posted here, will make the more learned members weep.
Hope this helps in some way.
AG
P.S. I chose the K98k. Which I now avidly collect but shoot mostly for pleasure. I've an Enfield and an M1903 for the competitions.
P.P.S. Imperial Mausers are a different story. There's a better supply of those in the UK, some of which are in excellent condition. But they won't have scopes.
P.P.P.S You can get K98ks competently re-barrelled here if you find a nice one that is just a bit worn - sacrilege to the collectors though.
