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
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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
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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.