If you can afford it, why not buy the best equipment to put you in a position for a faster return on investment? Sure, there are probably less costly options but at the end of the day, it's about ROI. Your business plan should clearly spell this out numerically. If you don't have a business plan, you are starting off on the wrong foot and chances to succeed are greatly reduced.
Is the invoice too much? Who cares? Does it align with your business plan?
Marketing is huge and should be factored into your business plan. It is as important as your hardware. I assume your skill set is matched by your equipment selection. If your business plan is coming up short on the ROI, look carefully at your marketing strategy. A creative, and strategic marketing plan will drive the quicker ROI.
In summary, the cost of your equipment is a moot point in a well prepared business / marketing plan for your career path. If the vendor is reputable and professional, the cost should reflect this. Again, if you did your homework and due diligence by creating a solid business plan, you should know that equipment cost should be viewed as ROI. Chase the ROI through a solid business plan. Marketing your skill should be able to drive the quickest return on investment.