Sniper rifles are a bad, bad choice for beginners. Sniping can be rewarding to certain types of player, but for most (especially beginners) it's uncomfortable, boring, bloody expensive, and a lot of effort. Sniper rifles are built, not bought, and a good one is a minimum of £350 plus tens of hours of tuning. Crap clone sniper rifles (e.g. WELLs, JGs, Warriors, UTGs) in particular can be virtually unusable out of the box; even good ones (Tokyo Marui) will be about as effective as a mid-range AEG but with a ROF measured in rounds per minute, not rounds per second. Forget the sniper rifle.
Pistols aren't bad but they aren't necessary. A good pistol is £125-150 plus another £40-50 for two spare magazines, another £25-50 for a holster, and another £10-25 for a double magazine pouch. If you add that (£190-250) to your primary weapon you could buy a gun that's really good, as opposed to 'just good enough for a beginner'. Unless you'll be playing a lot of CQB (and even then, it's not ideal) I wouldn't get a pistol yet; you have more important things to buy.
What you want is versatility; a weapon that will work reliably and effectively in CQB, field play, milsims, speedball, hot, cold, rain, snow - everything. For that you want something light, compact, and with a reasonable rate of fire - an SMG or compact assault rifle are your best bets by far. Common suggestions for new players are G&G Combat Machine ARs (£125-175) and CYMA cm.04x-series AKs (similar). If you want an MP5 ICS, G&G, SRC and VFC make good ones, although personally if you want something a bit different I'd be looking for a MagPul PDR, RealSword Type 97B or an AUG or P90 of some description.
This is extremely dubious at best.
Buying Secondhand: Just as with buying from a retailer, the seller, not the buyer, is the one who's liable to be prosecuted. Any seller with any brains will ask to see proof of your defence.
Modifying an IF to a RIF: This is a criminal offence - s.36(1)(b ), Violent Crime Reduction Act 2006 - just like importing, manufacturing or selling a RIF. The only advantage here is that it will be you committing the offence, not the seller, so at least you're not endangering anybody else.
TBH if you have records of going to a local club repeatedly you probably do meet the standards for a defence; can you not ask them to provide you with a site membership eligible for UKARA registration? It would save you a lot of trouble and possibly a considerable amount of money.