1. Build is severely unbalanced imo. If you can go for a 1080Ti sli, you can go for a better chip.
2. Yes, you do. Some high end mobos come with a HB SLI bridge, but many do not.
3. You cannot SLI without a bridge.
Like parani said, SLI is not supported very well nowadays.
You can do much better for the build.
With that budget, you can easily include an SSD and your case can be better.
There are better options:
8700k with 1 1080Ti:
PCPartPicker part list:
https://pcpartpicker.com/list/4p69r6
Price breakdown by merchant: https://pcpartpicker.com/list/4p69r6/by_merchant/
CPU: Intel - Core i7-8700K 3.7GHz 6-Core Processor ($327.99 @ Walmart)
CPU Cooler: Corsair - H100i v2 70.7 CFM Liquid CPU Cooler ($107.89 @ OutletPC)
Motherboard: Asus - Prime Z370-A ATX LGA1151 Motherboard ($149.99 @ Newegg)
Memory: G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($164.99 @ Newegg)
Storage: Western Digital - Blue 500GB M.2-2280 Solid State Drive ($109.99 @ Amazon)
Storage: Seagate - Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($59.79 @ OutletPC)
Video Card: EVGA - GeForce GTX 1080 Ti 11GB SC2 Video Card ($749.99 @ B&H)
Case: Phanteks - ECLIPSE P400S TEMPERED GLASS ATX Mid Tower Case ($59.99 @ Newegg)
Power Supply: EVGA - SuperNOVA G3 750W 80+ Gold Certified Fully-Modular ATX Power Supply ($79.99 @ Amazon)
Total: $1810.61
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-18 03:31 EDT-0400
2700x with 1 1080Ti:
PCPartPicker part list:
https://pcpartpicker.com/list/PKwW4q
Price breakdown by merchant: https://pcpartpicker.com/list/PKwW4q/by_merchant/
CPU: AMD - Ryzen 7 2700X 3.7GHz 8-Core Processor ($319.99 @ Amazon)
CPU Cooler: Corsair - H100i v2 70.7 CFM Liquid CPU Cooler ($107.89 @ OutletPC)
Motherboard: Asus - TUF X470-PLUS GAMING ATX AM4 Motherboard ($155.00 @ B&H)
Memory: G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($164.99 @ Newegg)
Storage: Western Digital - Blue 500GB M.2-2280 Solid State Drive ($109.99 @ Amazon)
Storage: Seagate - Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($59.79 @ OutletPC)
Video Card: EVGA - GeForce GTX 1080 Ti 11GB SC2 Video Card ($749.99 @ B&H)
Case: Phanteks - ECLIPSE P400S TEMPERED GLASS ATX Mid Tower Case ($59.99 @ Newegg)
Power Supply: EVGA - SuperNOVA G3 750W 80+ Gold Certified Fully-Modular ATX Power Supply ($79.99 @ Amazon)
Total: $1807.62
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-18 03:32 EDT-0400
8700k with 2 1080Ti incase you really want to SLI:
PCPartPicker part list:
https://pcpartpicker.com/list/vdLGLJ
Price breakdown by merchant: https://pcpartpicker.com/list/vdLGLJ/by_merchant/
CPU: Intel - Core i7-8700K 3.7GHz 6-Core Processor ($327.99 @ Walmart)
CPU Cooler: Corsair - H100i v2 70.7 CFM Liquid CPU Cooler ($107.89 @ OutletPC)
Motherboard: Asus - Prime Z370-A ATX LGA1151 Motherboard ($149.99 @ Newegg)
Memory: G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($164.99 @ Newegg)
Storage: Western Digital - Blue 500GB M.2-2280 Solid State Drive ($109.99 @ Amazon)
Storage: Seagate - Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($59.79 @ OutletPC)
Video Card: EVGA - GeForce GTX 1080 Ti 11GB SC2 Video Card (2-Way SLI) ($749.99 @ B&H)
Video Card: EVGA - GeForce GTX 1080 Ti 11GB SC2 Video Card (2-Way SLI) ($749.99 @ B&H)
Case: Phanteks - ECLIPSE P400S TEMPERED GLASS ATX Mid Tower Case ($59.99 @ Newegg)
Power Supply: EVGA - SuperNOVA G3 750W 80+ Gold Certified Fully-Modular ATX Power Supply ($79.99 @ Amazon)
Total: $2560.60
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-18 03:33 EDT-0400
2700x with 2 1080Ti:
PCPartPicker part list:
https://pcpartpicker.com/list/cm9gbX
Price breakdown by merchant: https://pcpartpicker.com/list/cm9gbX/by_merchant/
CPU: AMD - Ryzen 7 2700X 3.7GHz 8-Core Processor ($319.99 @ Amazon)
CPU Cooler: Corsair - H100i v2 70.7 CFM Liquid CPU Cooler ($107.89 @ OutletPC)
Motherboard: ASRock - Fatal1ty X470 Gaming K4 ATX AM4 Motherboard ($163.98 @ Newegg)
Memory: G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($164.99 @ Newegg)
Storage: Western Digital - Blue 500GB M.2-2280 Solid State Drive ($109.99 @ Amazon)
Storage: Seagate - Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($59.79 @ OutletPC)
Video Card: EVGA - GeForce GTX 1080 Ti 11GB SC2 Video Card (2-Way SLI) ($749.99 @ B&H)
Video Card: EVGA - GeForce GTX 1080 Ti 11GB SC2 Video Card (2-Way SLI) ($749.99 @ B&H)
Case: Phanteks - ECLIPSE P400S TEMPERED GLASS ATX Mid Tower Case ($59.99 @ Newegg)
Power Supply: EVGA - SuperNOVA G3 750W 80+ Gold Certified Fully-Modular ATX Power Supply ($79.99 @ Amazon)
Total: $2566.59
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-18 03:35 EDT-0400
As you can see, all of the builds are below $2600, and are much more balanced. They all have an SSD, better case, better CPU. There are also builds with 2 x 1080Ti's incase you want to stick with SLI.