First of all welcome to LRG which is already a huge step if you´ve decided to explore the world of Battle Beasts, there´s probably no better way to start than out here. There´s a lot of collectors around, although not all as active, all of which have been helping built this great place and thanks to all the input it´s a massive (active) archive of BB related topics. Have your tried the search function yet? This might help you straight away with the Rubsign topic giving you various insights and solutions that´ve been tried and tested out here over the years.
As with many collectibles there are, and always will be, various perspectives to certain topics especially in regards to fixing/replacing parts of an original figure which means you´re sort of restoring a 30+ year old figure. I´m probably one of the biggest purists out here meaning I´d like to leave the originals as untouched as possible and in their original state. For me the most important reason is to preserve the history of the line and with people switching rubs around or placing reproductions on original Beasts the history of the line becomes a polluted imho. But the fact that rubs might no longer work after several decades is indeed a problem. First of all it might be wise to be 100% certain if the rubs are indeed no longer functioning because just a simple rub might not alive work or bring them back to life. What you can try is to breath warm air close to the rubsign and in many cases this will do the job, I wouldn´t recommend using a hairdryer or other hot air solution since the chances of destroying the function of the rub by heating it up to much are also there. Once you have completely ruled out the chances of it working it´s up to you and your own perspective to make up the decision.
Will you go for the repair way in which way Stickerfixer seems to have been one of the most used solutions over the years. I personally wouldn´t recommend fooling around with replacing original rubs because the chances of damaging two figures, and or a still functioning rub, are way to high and it would be a shame if things get lost because of that. Some people might make it look/sound easy but the rubsign is made up out of various layers and it´s really easy to have one or more of them damaged meaning it doesn´t function the way it should.
Or will you accept the loss of the rub and thereby the functioning of the rubsign gimmick on that specific figure and still cherish it as such and go after a similar figure with a working rubsign? These days prices on Beasts seriously vary but that´s also mainly driven by the state/condition they come in, figures without rubs can still be found relatively cheap whereas figures which rub (although some have not always been tested so it´s not always said a figure with rub still HAS a working rub) are worth a lot more. Of course the Battle Beasts line is connected to this gimmick so for many of us the functioning of the rub is crucial.
There are multiple solutions and sides/perspectives to choose in regards to the rubsign so it´s up to you which might work best for you or with what solution you feel the most comfortable.
When it comes to rarity there are also various perspectives to look at it because in general the Series 1 Beasts are less rare than Series 3 Beasts because they´ve simply been more spread worldwide. The Laser Beasts, often considered to be Series 4, hold the most rare Beasts simply because there release was limited and the last 12 were even only exclusive to Japan making them even harder to find. But like Sydot mentioned there are also some Series 1 Beasts that might seem harder to find or are more pricey simply because they´re more wanted like for instance Pirate Lion which is famous probably because it´s the number one Beasts and it´s without a doubt the most recognizable Beast out there representing the BB line. Does this make him really rare, I don´t think so because he´s been produced in mass numbers but simply due to the demand it has become a more pricey Beast. This goes for almost every Series in which you´ll find some Beasts being more wanted than others. Among the normal 76 BB I wouldn´t really say there are ¨Rare¨ Beasts but you could consider some ¨Variants¨ to be more special than the normal Beasts. In the UK you probably might have the Orange Gorilla version around for instance which is considered a ¨Variant¨ or the normal Red Gorilla and you might also have the Full Shell Tortoise for instance where most of the other Tortoise releases had a stamp on the middle of the Shell on the backside of the figure. Here´s a short page about the most common Variants known but there´s still a bit of debate about them as well; https://beastformers...tlegs/variants/
The UK had all 3 Series although Series 2 and 3 have been available in a much more limited, probably regional, way and/or for a much shorter time so in general also in the UK it´ll probably be the most easy to find the first 28 Beasts belonging to Series 1.
In regards to purchasing Beasts it really depends a bit on what you want and what you are looking for, Ebay might work but maybe you´ve some local vintage shops or something around that might be worth a visit. But if you´re really looking for figures in a collectable condition it might be worth to check in with collectors out here and/or place a topic in the Need Toys section. I might be able to help you out with some figs as well although I´ll mainly have complete figs in good condition so I´m not sure if that´s what you´re after?
Check out the rest of the board I´d say and maybe you´ll find some interesting things on the Beastformers Blog as well and...
Enjoy the Beasts!