Hello there,
if you are talking about the cleats, they are not plastic, but metal. So they will last for a while. Also even for the metal cleats, eventually after some time you will have to change them as the everyday use will damage them. That is why the cost of the cleats is so low.

I did not get any with mine. Sounds like you would be better off with a platform pedal that has a SPD inside. Like Shimano M424 SPD Pedals?

They didn't come with adapters. The adapters I think you mean come on some new bikes, so you can test ride it in the shop without cleats. They're pretty flimsy, and I don't think they'd last long. If you're riding on the road or a gentle trail, these pedals can still be used with non-SPD shoes (just a smaller contact area!)

no, these are not included

No, nothing else was in the box.
But i have ridden them around the block with sneakers on and they are OK to ride without any adapters in them. Probably wouldn't ride any trails though.

My pedals did not come with the plastic platforms. However, many bike shops have a surplus of them kicking around. Go to your LBS and ask.

I bought mine from wiggle and they didn't, but i believe they usually do.

since you are going for cross country MTB, going for the better quality and do not think of the price. It will compensate in the future as it might never need to change them! Go for the XT

It's all about weight and type of bearing. The more you pay, the less the weight and the better the bearing. Even the chepaest Shimano SPD pedal will last for years though......

If you race you need these SPD's! The cheaper versions clog in our very sticky wet weather mud. They are well made and will survive daily grind commuting but step up to the plate when things get gnarly.

Hey Ian, as a first time user possibly not worth the extra coin. The M520's are adequate. I've recently upgraded to these after having M520's for 4 years. There was nothing wrong with the 520's, I put them onto the spin bike for rainy day training, the mtb got the new ones. I was going to get the M540's but the brother inlaw showed me M770's from Wiggle were half what we pay here (about the same price as 520's here I'd just bought the kids..), so I got him & me a pair. The only difference I thought I could feel was turning the pedal by hand while holding the spindle. The M770's seemed a little smoother. Weight wise I couldnt tell.

Would recommend the extra £'s for XT's as they clip in and out so easy. Would hate to get feet stuck in cheapies at a set of traffic lights!

I'd say yes they are worth it. It's really nice to have the cage when riding off road and the weight difference is so minimal I don't think it would matter for racing. I've used normal shoes and flip flops even when popping into town or somewhere like that.
A good choice for your first SPDs. Make sure you set them loose to start off with though to avoid toppling over with feet still attached.

Hi. I'm pretty sure you'd be better going for a pedal that has multiple entry. Some of these are road pedals. I would actually recommend Crank Bros for MTB races, as they are less likely to get packed with mud etc. and some are even 4 sided entry.. Hope that helps..

I have no experience of the XTR peddles but I don't think they could be worth twice the money (unless you take bragging more seriously than cycling!) My Shimano XT PD-M770 pedals have been fantastic. Never have to give them a second thought come thick mud or loose sand.