Some answers to your questions but first a question...
Why 18" snows when you have 17" summers? The lower the profile of the tire, the typically "worse" they are/can be under adverse conditions. What'd be more appropriate would be to wear down your 17" tires, replace them w/ winters and then get a new 18" (or 19") wheel pkg w/ summer tires and sensors. Plus, you'll have an easier time finding snows in 17" dia than 18" (and cheaper).
Anywho....
Yes, you can simply unbolt your existing wheel/tire combo and install the winter pkg. No special tools needed other than the key for the locking wheel bolts.
No real adverse issue w/ running w/o the TPMS sensors other than the constant ((!)) on the dash - though you may occasionally get an error message. Probably just on each start up which you can clear. Shouldn't come back but the ((!)) light will remain. I'm not sure how it is on the 9x7.2s but you may be able to reset the TPMS warning to 0lbs thru the PCM while you have the winter tires on. Check your manual for settings...
That being said, sensors, when bought as a set w/ rims/tires, usually aren't all that expensive (if you're getting the entire pkg from someplace like Tirerack.com).
Good luck
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