Seeing you picked the bike up quite cheaply now you can raid the piggy bank and start replacing the needed parts like tyres. Tyres don't come cheap for good ones, but I always think your life is riding on those two bits of rubber. I would never stint on tyres. You pay good money, get good tyres and they will last you a lot longer than cheap ones.
Get the bike up to your standard and there is no reason why you cant keep it for years.
You said the new exhaust system is making the bike look dirty. A bucket of warm water plus sponge and the job is done. Oh and a good polish after.
Look after the bike and the bike will look after you.