Well, the good news is that you appear to meet the financial requirement with no savings needed - provided you have been earning more than £18,600 per month for 6 months (which hopefully you will have by end September). Note that the UKBA will take the figure from your lowest month, so if there is a month where you were paid less than the required amount, you might need to delay your application until that particular month slips outside the 6-month consideration period.

You only need a solicitor if you envisage any real problems with your application (for example: if your wife has experienced any past problems with entering the UK or deportation, children are also involved or if there is any criminal record). Otherwise, you'll be fine applying yourselves and saving that money for visas or flights.

There won't be any interviews. Your wife will need her biometrics taken (photograph and fingerprints) but that's pretty straight forward.

Yes, you need originals of all paperwork - especially bank statements. That is very important. You say you only have 4 months, but you likely don't qualify as a sponsor yet anyway because you need to have been earning for a full 6 months (by then, you'll hopefully have all six months' worth of statements and pay slips).

Your wife's employment won't have any bearing on this application, though if she resumes work once in the UK, you can use both your incomes for subsequent visa applications.

You only need to send your wife a photocopy of your passport bio pages (page with photo and number) because the UKBA will be able to verify your information in their database.

Regarding your tenancy, if you are the only tenant, then I assume you don't need to worry about listing the number of bedrooms as there are only the two of you. Someone else will post if they believe you will, but I think you'll be okay without unless the documentary requirements state otherwise.