There are 4 options shown here. You might select one that fits your budget.
　　
　　The minimum memory requirement for 2500 is 16D/16F and for 2600 is 48D/64F. The suggested IOS is 12.2 Enterprise Version.
　　
　　　Option 1: The Best Option
　　
　　R1: 2514: 2e/2s
　　R2: 2620/2621: 2s/2Fastethernet/NM-1V/2-FXS
　　R3: 4500/4700: 1e/1atm/1serial
　　R4: 2503: 1e/2s/1bri
　　R5: 2610: 1e/1s/1bri/nm-1v/2-fxs
　　R6: 2501: 1e/2s
　　R7: 2501: 1e/2s
　　Console Access Router: 2511: 2s/1e/16async
　　
　　BB1: 2501: 1e or 1601 1e
　　BB2: 2501: 1e or 1601 1e
　　BB3: 4500/4700: 1atm/1e
　　Frame Switch: 2523: 10s
　　Cat1: 3550 EMI
　　Cat2: 3550 EMI
　　ATM Switch: LS1010
　　ISDN Simulator: Any brand, e.g. Teltone ILS 2000
　　
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　　Option 2: The Less Costly
　　
　　Everything as Option 1 except:
　　-Cat2 is 2924XL instead of 3550 EMI
　　
　　R1: 2514: 2e/2s
　　R2: 2620/2621: 2s/2Fastethernet/NM-1V/2-FXS
　　R3: 4500/4700: 1e,1atm,1serial
　　R4: 2503: 1e/2s/1bri
　　R5: 2610: 1e/1s/1bri/nm-1v/2-fxs
　　R6: 2501: 1e/2s
　　R7: 2501: 1e/2s
　　Console Access Router: 2511: 2s/1e/16async
　　
　　BB1: 2501: 1e or 1601 1e
　　BB2: 2501: 1e or 1601 1e
　　BB3: 4500/4700: 1atm/1e
　　Frame Switch: 2523, 10s
　　Cat1: 3550 EMI
　　Cat2: 2924 XL
　　ISDN Simulator: Any brand, e.g. Teltone ILS 2000
　　ATM Switch: LS1010
　　
　　-----------
　　Option 3: The Cheapest Option
　　
　　Everything as on Option 1 except:
　　-No ATM.
　　-Cat2 is 2924XL instead of 3550 EMI
　　
　　R1: 2514: 2e/2s
　　R2: 2620/2621: 2s, 2FastEthernet/NM-1V/2-FXS
　　R3: 2501: 1e/2s
　　R4: 2503: 1e/2s/1bri
　　R5: 2610: 1e/1s/1bri/nm-1v/2-fxs
　　R6: 2501: 1e/2s
　　R7: 2501: 1e/2s
　　
　　Console Access Router: 2511: 2s/1e/16async
　　BB1: 2501: 1e or 1601 1e
　　BB2: 2501: 1e or 1601 1e
　　BB3: 2501: 1e, 1s or 1601 1e
　　Frame Switch: 2523, 10s
　　Cat1: 3550 EMI
　　Cat2: 2924XL
　　ISDN Simulator: Any brand, e.g. Teltone Demonstrator
　　
　　-----------
　　Option 4 : My own recommendation -The Budget Labs
　　
　　
　　Where to buy?
　　
　　Ebay is the best place to buy. Look for the genuine and reputed seller with high credit rating.