I have many questions. I am used to knowing a reasonable amount about tax, getting the best out of utilities, contracts (basically the finances of running a house/life

) etc. so this is all new to me. I have used the search extensively and I apologise in advance if I am repeating a previous thread or parts of it. But I know, after lurking for a while, that this forum is a wealth of fantastic information... so here goes
I have a job in canton Zürich earning less than the 120K threshold for the self-assessment of tax. I am currently single, but will be married in August. My future wife will be moving out here in a few days. She starts work in August. Together we will earn more than the 120K cutoff. We will be letting our house out in the UK on which we are paying off a mortgage.
Am I correct in thinking that we will have to submit a tax declaration at the end of the tax year (is it March 31st here?)? If so I assume it is beneficial to have this done by a profesional in order to claim allowance for our UK house etc. - what sort of expense are we talking about in having all the tax stuff done for us.
Secondly, I first arrived and registered in Kreis 6. I have, after a measly 6 weeks, moved into a nice appartment in a lower tax zone (still in canton Zürich). I found that you pay the rate of tax you were on at the start of the year for the whole year. But I only arrived in June... will I move straight onto the lower tax scale or do I have to wait until January? Also is this just for convenience - and I can reclaim the excess tax (kreis 6 - lower tax zone) at the end of the year or am I really to pay more tax than my neighbours for 5 months ;( ?
Maybe I should have said already, but I am currently taxed at source - I lose about 25% of my pay to pension and tax. If I have to do the tax form, is it an adjustment at the end of the year rather than an altered monthly tax rate? How can I check I am being taxed correctly on a monthly basis - I really dont want the surprise of a huge bill t the end of the tax year

. I have tried the online calculators but I think these are estimates - or maybe not?
Thanks for any insight!