40 feet long x 50 feet width is not a problem for most 300Mbps 802.11n router. So you don't need a external antenna.

But the 35 clients become a problem. Irrespective of the bandwidth provided by your ISP, I think you need to use 1 router and 2 access point to share the burden.

As budget allows, I would like to use one TL-WR1043ND as the main router, add two TL-WA801ND as access point.

# TL-WR1043ND, (SSID_1, channel_1, area_A), which is supposed to supply 15 clients.

# TL-WA801ND_unit1, (SSID_2, channel_6, area_B), which is supposed to supply 10 clients.

# TL-WA801ND_unit2, (SSID_3, channel_13, area_C), which is supposed to supply 10 clients.

Access Piont like TL-WA801ND can associate with 16 clients, but only associate...To ensure the stable daily application for each client, better reduce to 10 clients for each AP.
Use different channels to avoid interference.

(((( 801 ------------------- 1043 -------------------------- 801 ))))