Switching to SCM Mid Year

Hello!  I have a 2nd grader, a kindergartener and a 2 year old.  We currently homeschool with a 2-day a week co-op that uses workbook style lessons and getting my 2nd grade son to sit down and do his assignments with a happy heart (or in a timely manner) has been extremely difficult and our homeschool days aren’t looking anything like I imagined.  We’re both miserable.  While I do acknowledge that part of the issue is work ethic and diligence (hello, habit training!), we’re both just starving for a living curriculum!  Our sweetest part of the day is when we sit down and read whatever literature book we are reading together.  So I am crazy excited to switch over to SCM!  My question is, do you recommend that I just start from the beginning with week 1 and go until however far we get at the end of the school year, or should I figure out which week it is and jump in there and just complete the second half of the program?  Also, do you all order all the required reading up front or as needed?  The co-op we are a part of is pretty expensive and we will keep them in it to finish out the year (they do love the two days there when they’re not at home) and am trying to figure out if I need to order all the reading books at once or if I can space them out… I’m trying to space out the spending.  Thanks for your help!  I’m so thankful for this community and for this amazing curriculum!